What time of day would you be arriving into IAH or DFW?

IAH is slightly closer to the Austin area, depending where in town you're headed. That said, there are toll roads, both in Houston and Austin that you'd probably use, so factor that into a rental car cost (if you're renting). There aren't highways the whole way between Houston and Austin either.

DFW is further, again depending where you are headed. There are some toll roads, but they can probably be avoided. It is a straight shot on the I-35. However, there is almost always construction somewhere along the route, and it is very heavily traveled.

My advice....if you'll be driving mostly during the day, and avoiding the worst of the rush hour (especially leaving either DFW or IAH), then fly into IAH. If you'll be driving at night, then use DFW. I've done both, and neither is very fun.