Initiation and spells, mostly. Foci, I try to get the GM to lt me pay my enchanter contact the 5000 /karma to bond a focus to me. Also to raise to 3 whatever Attribute I gimped so I could max out another.

You can allocate Sorcery dice to help with drain. You also allocate them to spell defense. At some point you have to worry about Focus addiction, which leads to at least one reason to raise your magic Attribute. Magic Attribute also determines if th Force of your spell od spirit is going to cause Physical or Stun Drain, it also limits the Force of the Spirit you can summon.

Magic rating is important for Banishing Spirits. Magic Rating determines how many Foci you can have active without rolling for Focus Addiction.

I would have to check concerning SR2, as Sorcery Skill was not directly rolled when casting spells, Spell Force was the primary dice pool.
Actually I would check all your questions and my answers against SR2 rules. I was answering only concerning SR3.

The only way I can think of right off for other than the owner to deactivate a focus, other than destroying it, is to remove it from physical contact with the owner. The owner can deactivate as a free action.

A magician can Dispell the spell in a sustaining focus or spell lock. IIRC in SR2, Dispelling was a metamagic, in SR3 it is a specialization of Sorcery.

I agree, my desire for Initiation is to get the metamagics. Usually I would be glad to trade a second metamagic for the +1 to Magic, if it were allowed.

Summoning Big Nasty Great Spirits™.

Ahh, Invoking. I was hoping maybe there was another obscure metamagic.

What edition are you playing? This is kind of the default SR3 forum, but it seems every time I act out of that belief, I am wrong.

I like Shielding better than Absorption or Reflection. It seems too difficult to get Absorption and Reflection to really pay off, but Raising TNs by your grade can quickly be real effective.

I like Centering, too, and usually have a Knowledge or Language skill allocated for it when building the character. Centering can reduce drain, give extra successes, or at a decent Grade, can really bring down TNs.

As a Sorcerer I would probably go with Masking, Shielding, and Centering.

@ the original question, and reminded by a prior post, I once gave karma to Buttercup (I was in puppy love with her/Bacchus shaman and enthralled by her.) I used the conjuring ritual, trying to increase the exchange rate (but really trying to figure out what Force the GM was assigning her in the hope I might astral quest for a true name and bind her. )

The only thing about getting masking as a level 1 initiate is that it really only works on non-initiated magicians. I think the way the rules work, the GM makes a test when a mage tries to see if you're magical. I'm not looking at the book right now, but I recall it being something like the GM rolls your magic attribute number of dice against a target of his initiate grade, and then his magic attribute number of dice against a target of your initiate grade. If he gets more successes than you, he sees through your masking. Definitely a meta-magic that gets more powerful as you initiate more. I kinda always liked shielding - it's a nice beginning utility spell that keeps a bunch of people safe. YMMV though.

Big, powerful expendable spell foci are your friend, though. They're pretty cheap compared to what your buddies are gonna be buying, and 10 extra dice on a spell when you need it is just plain boss.

In terms of spell defense, it cannot be stressed how unbelievably superior shielding is to anything else.

It's been awhile, so maybe my memory is rusty, but as I recall: Allocate X dice to shielding. Select X targets to be shielded. Each target gets X dice to resist spells. Spells cast at target incur +X to their target numbers. X cannot be greater than your initiate grade.

That's pretty much an "I WIN" button against spell casters. Or at least a reason why a little skill in something other than sorcery is mandatory.