Q is more formulated for getting low and sounding really clean. The SX line of subwoofers were designed SOLELY for use in ported enclosures and they will get stupid loud doing so... If you're looking for pure loudness (and have a decent amount of EXTRA power on tap) then go with the SX.
P.S. I hope you mean a REAL SX, not that ***** *** SXX that RE is making now. Find the older shit with the traditional 12 spoke basket and whatnot. The newer lineup of their subs with the extra X tacked on the end of the name are kinda meh...