AUS commuter here. For schedule flexibility, AA is a stronger bet. DFW is a bit farther, but AA typically has 13-15 flights a day to DFW, making commuting by plane substantially easier (that said I don't know what AA's reserve rules or commuter policy is). AA is also by far the #2 carrier in AUS. If you're the type of person to use your benefits a lot, AA seems to go just about everywhere out of AUS these days. UA has 6-8 flights a day to IAH. There are a couple of big gaps that make commuting by air less than ideal, but I still do it.

As some data, any trip that reports after 8:30 is commutable on the front end. Sure, you have to be on the 5:30 AM flight, but that's not bad. Usually if you end before 9 PM you'll get home that night, sometimes even as late as about 9:45. So the overwhelming majority of trips are commutable. Plenty of redeyes from the west coast that get you I will early enough to take the first flight back home.

Many will just say drive, but I just really don't like driving. I hate getting back from a trip and having to stay engaged and alert for 3 more hours. I live really close to AUS so the time is usually a wash when you consider having to wait for the next flight, but it's just easier for me. At this point I'm accustomed to spending time in airports.

As far as reserve goes, I wouldn't chance it, especially in your first year. Most places in Austin are closer to 2:45-3:00, though it can be done in 2:15 if you speed/avoid traffic. But don't forget you're not in position at the employee lot, so you also have to factor the time to park your car, get to the terminal, and get to your crew. You can always drive partway. There's a Starbucks in Giddings, a Bucee's in Bastrop that one could post up at. Or just aggressively pick up stuff, which is what I did. I only sat one day of reserve.

I'll just say in closing that United has been great and the commute is really straightforward. Just knowing that you *can* drive is a great peace of mind.