what's the output of the RF? the MTX is 1500x1, so 75o watts per sub. personally I'd go with the MTX just because I think it's a better amp. I'm not a big fan of RF's newer stuff.
Make sure you run a ported enclosure with the S15L7's if you're looking for max SPL. HUGE difference when going from sealed to ported, these subs like cu ft. Although a bit sloppy, they should get plenty loud.