Hi shawn! I just spent the last year living in Montpellier and could give you LOADS of specifics on all 3 places you suggested :) What are you interested in?
Montpellier is probably best for a few day stay--it's very lively at night and has a surprisingly "international" feel. It does depend on the time of year you're visiting though. It's also kind of dirty (in a southern city kind of way), but loads of fun. The people there are great--very friendly and open, and love living there. There's tons of bars and restaurants.
Sète is cute. I went there several times. I really enjoyed it, but there isn't a lot to do there. It's good for a day or maybe 2 day trip. Of course it also has the beach, which can eat some time :)
I LOVE St-Guilhem. It was the one place I took all my visitors. That being said, the vibe varies greatly on day of the week and time of year. It's good for a half-day trip, or potentially overnight if you want a very quiet evening.
You didn't mention Nîmes but it's also lovely--interesting history. Kind of quiet at night, but still makes for a good overnight or 2 day stop.