First off, thank you for a well made argument. Let me address some of your points.
QUOTE (Fatum @ May 25 2013, 02:04 AM)

First, roleplaying is a collective pastime where every player is supposed to get their time in the limelight. Usually, the time they get is directly influenced by their characters' abilities: the more capable the character, the more often his actions are crucial to the plot, and the more often the player gets to enjoy the limelight. To take it to an extreme to illustrate: if you have two players and the first one's character is a godlike being of immense power, and the second one's a measly mortal incapable of influencing the world around him due to being quadriplegic, in any plot based on classical storytelling techniques, the first player will get all the limelight, while the second will at best be able to describe his character's internal monologue.
So according to this truth, the most powerful character is the one with the most time in the limelight. Technomancers and hackers then, because their respective rulesets take very long to resolve their specialties? Followed by mages on astral patrol?
Or rather the characters with the most IP in combat heavy adventures?
Sorry, I am not buying this argument in its entirety.
Your example is a good one, though. It is entirely possible for two players to build a highly optimized god-mage and a quadriplegic cripple. Hopefully the cripple will have some reason to sit on the table (maybe he's the hacker?), at which point it is the GM's job to ensure their time in the limelight for the cooperative storytelling that is an RPG.
If the GM allowed such characters to be created, he is responsible to make sure that their respective specialties are used. God-Mage will very probably not be the matrix-nut, so it balances out.
If the GM does not do this, he failed at his job and one of the two players will probably go home unsatisfied. Except when he is kind of a masochist that way

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Second, character archetypes exist for a reason, and they're based on works of fiction/other media as well as our real world experiences. As such, a street samurai is an archetype present in the genre since its very beginning, and consistently showing up in Shadowrun's editions, as well. Thus we can presume there will be players willing to play one, and I bet there will be street sams among the pregens.
I don't really see your point here. Could you elaborate?
If you mean by that that the mere presence of an archetype skews the perception of power levels, I agree.
I agree that people will play a street samurai, without thinking "Will I be weak?" Because they generally do not need to compete with the player sitting next to them playing a mage. Why? Because the GM makes sure that both get to enjoy their stuff.
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Now, let's compare two players, one playing a mage and the other a samurai, and see how capable their characters are. One can't get anywhere secure openly, because he's full of cyber and needs a weapon to be effective. The other can mask his nature, and even the rare measures capable of defeating that disguise are far from sure-fire. One can kill about two opponents per IP, if they are not too armoured, and taking out anyone capable of soaking damage is taking him a few times longer than that. The other can not only kill any two opponents in a single IP, he can turn them against their erstwhile comrades. Regardless of the defensive measures they employ, because there are no effective countermeasures against his attacks. He is also doing that while flying, invisible, protected by lifelike illusions and surrounded by summoned helpers each of which rivals a sam in stats. And he also has a whole layer of reality as his personal playground. And he doesn't get any of the social stigma, or the mental trauma, or any of the maluses, really, to boot.
This example is, in part, a fallacy.
To demonstrate my point, I'll deconstruct your example, at least for a bit.
First off, there are devastating samurai builds with unarmed combat, often as a backup to the weapon-necessity. They can very well get anywhere in security in the right environment. The highly cybered street samurai wouldn't be out of place deep in the Redmond barrens, or a combat zone. The prissy mage would draw fire. The street samurai doesn't need to wear his cyberware openly and thus can also mask his nature. The mage needs to initiate, before he can do this. The mage can run into background counts which makes him very unlikely to be that combat monster you're describing, while the samurai will still have his wired reflexes and a big gun. Cover provides penalties to some spells, why is counterspelling not an effective countermeasure and how many sustaining foci is that mage carrying?
And so on.
Why am I making these things up? To show the futility of it all.
We could go round and round for hours, if we were so inclined and find arguments and counter arguments in an endless circle.
My point is, it depends on multiple factors, who is actually superior.
For one: Black Trenchcoat vs Pink Mohawk. Where is the group on that scale? For two: Situational modifiers. For three: Individual character creation and progression choices. There are also in-setting social stigmata against mages, which you conveniently ignore. People are scared of magicians, ever since the Ghost Dance. If you let that just fly by, it's your fault as the GM. There are possibly more factors which all play into that particular equation, and I don't think it's just putting in a few variables to get a universally applicable result.
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Which brings us to Stahlseele's point dismissed so casually: a mage is capable of doing anything that a sam can do (except better) plus so much more. This has nothing to do with "powergaming" or "optimization" (which are for some reason used negatively, despite being just roleplaying competence) per se. This is just a question of narrative balance, and the balance of narrative focus upon different players' characters. When one of the archetypes is in all the meaningful ways better than the other, you can't dismiss that concern by just stating that they are different, since their competence areas overlap.
Oh, it wasn't so casually dismissed. I very carefully considered it before dismissing it.
If you got the impression that I used 'optimizer' as a derogatory term, I apologize. I tried to use one devoid of judgment, since I actually don't care about how much a person optimizes. I'll call out 'cheesy' (this is of course, depending on table and preference) options, and really bad concepts likewise. Where I disagree is that optimization represents a 'roleplaying competence'. This is not true. An optimizer can be a bad roleplayer, just as a 'true roleplayer' can be one. Numbers do not have anything to do with the ability to roleplay, just the ability to 'rollplay'

To address your actual point: Yes. A mage built to do the thing a samurai can do can do the same things, and, depending on a few dice rolls, presumably even better, until he gets to an area with background count. See above.
It is entirely viable to build a mage or an adept to mimic things a street sam can do. There is still no balancing involved between the two characters, because it is the GM's job to use his skills for creating a balance of narrative focus (I like that term). In 20 years of roleplaying and 15 years of being a GM, I haven't had a problem with street samurai players complaining about magic players. Usually it was the other way round.
"He has sooo many actions!" - "Then why don't you learn improved reflexes as a spell?" - "Oh good idea."
The thusly mollified mage player did not, in the majority of cases, use his new found quickness to splat enemies all over the place and instead focused on utility. And even when he threw mana balls around like there's no tomorrow, the street samurai players seemed happy as long as they could shoot things and demolish people.
This also neatly addresses your remark about competence areas. Of course competence areas overlap. Redundancy is a good thing, and players usually realize this. You have the samurai and his main field of combat, and the mage throwing his mana ball or stun ball or whatever floats his boat and degree of smart optimization (yes, stun balls

). Is the samurai unhappy that he's got help? I think not. It is not a matter of who's best. It's a matter of "Did I get to do my thing?"
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That is why there are design choices like limits on spirit summoning and service areas or mentor spirits affecting the magician's personality, after all.
I don't really follow here. Are you saying that mentor spirit personalities are in the setting because of balance against other character classes?
You've got a good point though about the limits on summoning. An army of spirits is nothing to laugh at as it is.
However, I am not entirely sure if that limitation is part of the internal balance of the field (which I mentioned earlier) or to balance it against other character classes. A bit of both, most likely, which means that I'd need to concede that there is a ceiling to the absolute power of the mage.
Except there is none. As mages progress in karma, they linearly progress in power. Street samurai go broader with buying secondary skills, mages just learn and learn and learn.
Yes, I freely admit that this can be a problem in extremely long campaigns. From a gut feeling, I'd say 7+ years.
Why?
Because I've played in a 7 year long campaign where I played a cybered mercenary. Our mage was initiate grad 6+ and had a ton of spells. I never felt inferior to him. In fact no one felt like they played second fiddle for his spotlight hogging antics. Because we had a good GM, who instinctively created a balance of narrative focus, instead of focusing on the system-inherent lack of balance between character classes.