How is the vocal talent "working" the mic? Are "they" handling the mic in the studio, or can they work it if it is boomed?
If they can handle it boomed, I would say go with the SM7 because it is a good, dry mic that is particularly impervious to the plosives that a hip-hop artist can dish out. The Sennheiser is exactly the opposite. I love that mic for a lot of applications, but you'll need to BURY it in a good windscreen!
The RE-20 is probably the most versatile of the bunch. Unlike the other 2,
it has virtually no proximity effect, very little handling noise if you DO go the handheld route, and it is bullet-proof... though you will probably need to put a Windtech screen for it, as well. I am a big fan of the RE-20, and have used it for SO many situations over the last $%^$ years!

Stick with the SM58. It can't be beat. The SM 7 is just a glorified 58 and I think would be of no real advantage. Just try to keep them from cupping their hand around the back of the microphone as it makes for really trashy sound and screws with the directional pattern.

Push it in the bush
Ms. Remy Ann David

It's all a matter of opinion ofcourse,but i'm very sure the very nice vocal sound brendan o' brien got on -amongst others- the last incubus album, is very impossible to get with an sm58..
I personally hate sm58s, but looooooove sm7's..
(There's also sm7's on some of ben folds vocals, stone temple pilots, and ofcourse the chili peppers, although in the last case the sound is so thin and thrashy, any dynamic could have done)..

anywayy.. I'm an sm7 fan..

i recommend the re-20. that is a very wide microphone. it never seems brittle or harsh with anything. it has a lot of uses.
i don't like using the sm58 for recording , only used live situations.