SM7 is the best HipHop mic period...hard rock music too...alot of kids see a LDC and think they have to use it...but they are only suited for certain voices like clear tenors and women without raspy overtones...the SM7 will win on almost everything else as would some of the other dynamics.
They both have their place. On a specific voice, either might sound better. That said, I generally like the SM7 more often for vocals than the RE-20. The RE-20 gets used more often for kick, bass amp and horn duty. SM7 gets used more often for guitars.
FWIW, I own 3 Sm7s and 1 RE-20 if that gives you an idea of relative application.