I wouldn't call myself a "shopper" (unless Amazon and newegg counts) but I grew up smack in between both of these locations and have been to San Marcos on a yearly basis for longer than I'm willing to admit. While I have no experience with any of the outllet malls in the Houston region, I'd say you would have to be crazy to drive past them just to get to San Marcos.

I would be beyond surprised if you couldn't find everything and you were looking for and more within 50 miles of IAH (or HOU f.t.m.) . If all you want to do is shop... don't bother driving 150+mi.

Now if you have some extra time and want to see some sights like the Alamo or riverwalk (San Antonio) or something in Austin (ie 6th street) then the drive is absolutely worth it. There's plenty of touristy/locally stuff to do throughout the central Texas region. Please forgive tthe lack of foliage, it has been a hot -HOT- summer and normally we keep things aa bit more green.

Whatever you choose, I'm sure you will find what you are looking for. Welcome to Texas :-)