We live in Tx, too!
Back in 2002 we went the second week of March for Spring Break. The weather wasn't bad at all. The first day there my niece and sister even more jeans. (I'm not saying it was cool, however. Just not sweltering). Our second day, while at MGM, it was hot from the get-go. So hot that my sister had to buy a shorts outfit for her daughter while waiting for the rides to open. I remember that one day being the hottest. For the most part it was pretty nice. It was nothing like summer, although I imagine some days are hotter than others. Just the luck of the draw.
This year we arrive on October 17th. Hopefully the weather will be nice then, too.