Both are excellent mics and both are quite versatile, and both I would consider "gotta-gets" for any mic locker.
If you're looking for a lot of floor tom, guitar cab and brass/horn work, you might want to lean towards the 421. If you;re looking for something a bit more vocal, drum and acoustic instrument application, the RE-20 might have the advantage for you.
Also, keep in mind that the RE-20 usually prefers preamps with a bit higher clean gain range, so if you're only running something along the lines of an inexpensive Lexicon or mAudio interface for your preamp, you might expect lower record levels out of the RE-20, and maybe the 421 might wind up being a bit more versatile for you. If you have some halfway-decent preamps with plenty of clean gain, though, all bets are off in that regard, and the RE-20 might have the slight edge.
But either way, you should get plenty of happy use out of either one of them.
G.