Justin,

I have used both mics. Both are good. I prefer the 604 over the SM57 for a few reasons. First, a engineer I respect recommended them to me (he has a Grammy, so I trust his ears). Second, if you can't afford the 421, the 604 has a similar sound (to me, the 604 has a more "open/bright/airy" sound than the 57). And third, having recorded quite a few rock drummers, I have had more than a few 57's destroyed from stick hits. The 604 uses a fiberglass shell which can take direct hits (and I can verify this) and not hurt the mic. Finally, every magazine that has reviewed the 604 seems to have liked it. So I can say without any doubt, go for it!