Hi Roadrat,
I don't live in the Houston area, so someone else will have to answer the first question for you. I can tell you that where I live (East Tennessee), AT&T has more consistent coverage, but for my everyday travel, T-Mob works fine, and is who I use.
As for getting a SIM card, a quick trip to the carrier store is in order. You'll want to ask for a Micro SIM for a new prepaid account. You can always get those online as well. You won't need any adapter of any kind, you'll just need to get Micro SIM instead of a full size.
AT&T does offer a month to month prepaid, but (again, in my area) the $65 smartphone plan with AT&T offers you 1 gig of data, with everything else unlimited. For $60 from T-Mob, you get unlimited everything, with 2.5 gigs of fast (HSPA+) data. Another $10 will get you unlimited fast data as well. Personally, I would not even consider AT&T, but depending on the coverage in your area(s), you may have different needs.
I hope this helps.