Despite its low ranking, firms in Houston seem to really like South Texas. Elsewhere in Texas, you would start to face blistering competition from Texas, SMU, and Baylor. (Not that those schools' graduates aren't applying in Houston, too, but folks there are more familiar with South Texas.)
I don't know anything about St. Mary's, so I can't definitively answer your question. But I think South Texas could be a good option if you plan to stay there.