Hi there! I actually attended UT Austin for CE undergrad, and now I am working on a PhD in trans.
As for your concerns:
TAMU- Growing up in Houston, I spent a lot of time in College Station for athletic tournaments and conferences in high school and college. It was a very boring place to be and maybe that is why they can afford to pay people more. You are right about TTI though- it is huge and they are internationally recognized. You can't read a single report that doesn't have some part of their research in it- though they are very focused on the more technical stuff: vehicle and driver characteristics, human factors, congestion pricing and that sort of thing.
UT Austin- I really enjoyed my undergrad days here, but I didn't want to stick around for 5 more years in the same town. The one thing that bugged me about Austin is that while it is a great city, pretty much everyone acts like its the greatest place on earth. It got really annoying to me for some reason. Anyway, UT does accept a lot of people right out of undergrad, so maybe you just didn't meet some of the younger students, but trust me, they exist. Also the engineering part of campus is right next to some cheap bars where people go to get drunk at 3 pm. So don't worry about a lack of community, it's there. Plus there are a lot of fun things to do in Austin- hiking, biking, running, kayaking, bouldering, music, food etc etc. UTs trans faculty are also doing more travel behavior/land use type things.
of course both schools have profs who advise congress and other policy makers. And you know, UT's CTR and TAMU's TTI collaborate a lot on research- they have a lot of post docs who work together.
All that being said- I think you should make your decision based on the research of the faculty who you want. If you are still in school and have online access to academic journals, look them up and see what they are doing that you are interested in. A quick one would be "web of science", a database that most universities pay for. The best 2 pieces of advice I ever got for picking a school:
1. Find the person who is doing exactly what you want to be doing, and go work for him/her.
2. Find the person who is doing exactly what you want to be doing, and go work for his/her old advisor (or go get the same training at his/her PhD alma mater)
That may never actually help narrow the schools... but I at least keep it in mind as a quick answer for when people as me why I chose my current school.
Good luck!