Whatever you do, you'll need substantial trapping in the corners to deal with the offending modes created by the square shape of your room. And by all means budget to included absorbers at the first reflection zone (even with your mix position) and possibly a couple clouds above the mix position. 2" 703 is only $1.50 a square foot, so spend a little more and make sure you hit all the problem areas. My last studio had a 20x20 control room with a 10' ceiling...half a cube, terrible bass response, so we ended up resizing the room to 20x17 and that little change (plus all the trapping we installed) made a world of difference in hearing what was going on in the low end. I prefer fiberglass to mineral wool because it's easier to work with, cut, and cover plus easy to glue together to make thicker panels for bass traps. Go check out Ethan Winer's site (
www.realtraps.com), some great reference material there for treating any room.