USC doesn't rank if you're worried about being in the middle of the class.
Anyway, I'm not sure that either has much of an advantage in national portability. I think that it's likely you'll have more options being at the very top of the class at Texas than you would at USC, but after the top 5% or so, it probably evens out. On the other hand, I think that students toward the bottom of the USC classes seem to have better options than those at Texas (perhaps as a function of being such a smaller class). But those are just the thoughts of a prelaw student.