I second and third what these guys said.
I own both, and I appreciate the mc012 for the added options the interchangeable capsules (and pad) bring to the table.
Other than that, I think of the 603 as being a little bit brighter. This can sometimes be problematic on overly-bright cymbals. My preference is for the mc012 on overheads, but I prefer the 603 on accoustic guitar almost every time. They are pretty similar, though, and either one should do the job for you just fine.