Essentially...
T-Mobile
PROS: Reasonable pricing, fast HSPA+ (where you can get it)
CONS: Poor (and in some cases, nonexistent) rural coverage if you travel
T-Mo is a good deal if you're going to be staying in the city (you mentioned you live in SoCal) for most of your usage. Just don't expect stellar voice coverage or anything faster than 2G/EDGE speeds as you get further away from civilization. And FYI, you get roaming with postpaid T-Mo, but not with prepaid. So that's another thing to consider.
I currently have T-Mo prepaid ($60 plan) on my G-Nex. T-Mo's coverage is pretty good where I live (Phoenix) and I don't travel a lot. When I signed up, they also gave me a $5 discount promo for the first 6 months, making it $55/mo. Not too bad...
AT&T
PROS: Extensive nationwide coverage area (good for travelers)
CONS: Slower HSPA+ in most areas, overloaded network spectrum in some metros (like NYC and San Fran), expensive
Honestly, I'd only recommend AT&T to those who truly need the wider coverage (frequent travelers) or simply live in an area where T-Mo's coverage won't cut it (this shouldn't be a problem in LA, though), because they're a lot more expensive.
Another option would be checking out an MVNO like Straight Talk. You can get either a T-Mo or AT&T SIM from them on a $45/mo. unlimited talk/text/data (2-3GB soft cap, by most reports). The only notable downside with Straight Talk is that their data use policy is pretty vague (they're not up-front with their actual limits). They sort of arbitrarily decide when they feel you're abusing data and they'll warn you to stop or threaten service shut-off. Ultimately, I went with T-Mo prepaid because at least they're up-front about what you get and they simply throttle you after your high-speed data limit is exceeded.