Two subs over one, Rythmik over REL. I have both. In my opinion, REL is good but overpriced, given (a) the flexibility of control available on Rythmik (adjustable phase, especially) and (b) the premium paid for REL. REL fanboys will disagree, but having completely solved the bass in my room (using analyses of room modes, placement, and every possible adjustment), there is no substitute for the control offered by Rythmik.
Compare:
The *adjustable* phase on Rythmik and the various toggle switches make a huge difference for dialing in subs and getting the bass correct.