Can't speak to Texas knife law, but as far as the knives themselves go, there are generally two types. Automatics that open by flipping open in an arc from the handle, like a conventional knife, and those that open by the blade coming straight out the front of the handle. I think the latter is easier and more secure to use, as you don't have to worry as much with finger placement when activating the blade.

Look for a model with a recessed activation button (proud buttons can be depressed in your pocket by bumping against something). Some have safety switches as well.

I own three or four automatics, but find a well designed assisted opening, or flipper knife to be just as quick, and usually less expensive for equal quality. Maybe more reliable as well? Automatics are cool though. Try out as many as you can before purchase. Look for something that carries well, feels good in your hand, and can be easily closed with one hand. I have always been partial to Benchmade knives in general, but if you can find a clean, used "GT" auto (I believe they are now out of business), they are a heck of a nice automatic.

Larry