I'm working on a concept for a custom-tradition urban shaman. I would like to modify the Aspected Magician quality from Street Magic (p. 26) to reflect the fact that he can't really use his mojo outside the city. I've come up with a couple of options:

A) Location (City) Aspect: The character takes a -4 dice pool modifier to all magical tasks (spellcasting, conjuring, assensing, etc.) when not in an urban or suburban area.

B) Location (<specific city>) Aspect (e.g., Seattle Aspect): The character takes a -4 dice pool modifier to all magical tasks (spellcasting, conjuring, assensing, etc.) when not in the Seattle Metroplex. If in another city, the modifier is reduced to -2 (reflecting fundamental similarities among all cities).

What BP bonuses would be appropriate for these qualities? Is option A strict enough to qualify for the 10-point bonus, or do they conform to the 5/10 standard given in Street Magic?

I think you'd be better off with a geas of 'can only cast spells in the city'.

I considered that, but I think the aspecting penalty makes more sense for the character. Of course, nothing is stopping me from taking both an aspect and a geas, which I have also thought about. There's also the fluff issue. An aspected magician's power is actually skewed in some way, whereas geasa are presented as being at least partly psychosomatic (psychoarcane?): you only think you can't use magic under condition X.

While I can certainly see the aspected as being a 'physical' limitation and the geas as being a 'mental' limitation, I think there is also room for interpretation as it being otherwise. An aspected mage could just have a mental block towards summoning spirits or whatever. And a geas might be 'mental' but that doesn't make the loss any less real, especially since it can become permanent and thus never mentally overcome, becoming 'physical' really.

I also figure that a city mage would be more likely to have a mental based dependence than a physical one. I don't think you'll have enough of a difference in magic between Chicago and LA to have it actually affect casting a spell, or even enough of a difference between a city and wilderness (Not counting pristine wilderness). Instead I can certainly see someone who is uncomfortable when not surrounded by concrete and tall buildings, and that discomfort translating itself into a difficulty focusing on magic.

Just seems odd to think the magic around a particular city would be so different from that around another city that a mage from one city would have such extreme difficulty in another city. And the applicability of a 'Seattle Aspect' as a proper flaw depends somewhat on the campaign. If there are no plans to ever leave Seattle, it isn't much of a flaw. If it is a gloabtrotting game, then it could be a serious drawback worth more than just 10 BP

Let the Geasa function like a background count instead up to -4D6.

City: 0
Suburbs: -1
Outer Suburbs: -2
Countryside: -3
Wilderness: -4

The further you get from a city centre the less effective your magic becomes.