ooh, now that's a tricky one.
I've heard both do great jobs.
I think I might be tempted to go for the Beta 98's, because I love condenser mic's on the toms, plus they are cheaper than the MD421's.
The only thing I don't like about the 421's is the "clip"/mount. I always hated that thing - but those are incredible mic's.
If you are buying these for a home studio, I might have to say go for the Sennheisers, because they can serve more than one purpose. I wouldn't want to use the 98's for other things.
Tim