This is a Sprint network focused site, so understand that you are likely to hear the benefits of Sprint. Plus, AT&T is a terrible, rapacious company that you should patronize as little as possible.
In Sprint's favor, its LTE deployment, once complete in Orange County, should be superior in many ways to that of AT&T. In all markets, Sprint is initially deploying 5 MHz FDD bandwidth. AT&T, on the other hand, is deploying 10 MHz FDD bandwidth in some markets, 5 MHz FDD bandwidth in others. Los Angeles is one of those others.
So, in greater Los Angeles, Sprint LTE and AT&T LTE will both be on par for LTE bandwidth, but Sprint likely has fewer subscribers yet greater cell site density. And that could add up to a Sprint advantage in LTE speed/capacity.
AJ