I've got an RE38, which is a sibling to the RE20. It's a wonderful vocal mic, great on guitar cabinets, and nice in kick drums (although i tend to prefer the beta 52 for that these days). Very similar sounding to the RE20.
If i already had an SM7, I would look at either the aforementioned heil pr40 or a sennheiser 421 instead of the RE20. Hard to beat the 421 on a guitar cabinet, and it's nice on kick and some vocals too. However, with the shoes reversed (already owning an RE20), I would put the SM7 at the top of my list.
That said....it seems if the OP is looking for a mic to handle primarily vocals and low end duties (kick, bass amps). If that's indeed the case, stick with the SM7 for vocals and get a Beta 52 for your kick and bass. Your mixes will thank you. Use the SM7 on the snare or a tom.
cheers,
wade