Well, I live in El Paso, which is pretty much half way. If you are looking to spend the night somewhere on the way, this is a great place. TONS of good cheap hotels right off of the freeway. If you just plan on stopping to eat, make sure you stop here to get some Mexican food. IMHO, it is the best! I'm sure you're aware of this by now, but there is no need to get on I 35 to get to San Antonio. Easy trip, I 10 all the way. I'll be making the trip as well (about 8 hours for me) so maybe I'll pass you on the way B)
By the way, if you want some suggestions on good places to eat in El Paso, just let me know.
Good luck,
Doug