I went with a used REL in the ST line (Stadium, Strata, Storm, etc). And I love it. Mine is a Storm III.
I would stay away from the REL T series with the Square Metal around each side (like the T1). These show up with the sub cabinet broken away from the metal squares somewhat frequently and REL abandoned this design in the most recent T5, T7 and T9.
Otherwise, I'd go with the size that works best in your room and the frequency extension that you are looking for.
MusicDirect sells REL subs, and they have their great 30 day return policy. Might want to give them a call to get their advice on the current line, if you are thinking of buying new. Or find a local REL dealer.