I would definitely recommend going to San Antonio or Austin over Dallas.. especially if you have a limited amount of time in each place. Those cities have some cool things to see, and you could do either in a day. Also, sites are more "condensed" so you could do a lot without having to travel all over the place. And with San Antonio especially, you could do it all on foot.
I'll have to think of my list of things to do in Houston... Warning: it may be long
