The Beyer compared to the 421...
I have owned a number of Beyers over the years: M500's, M260's, M69's, and still have an M99 and an M160/130 set. And plenty of 421's have passed through my hands. The M260 is a great-sounding mic on snare, brass, and many vocals. Very smooth and detailed, very tight pattern. That's the good news.
The bad news is that they are not very rugged, don't like high SPL's (they ARE a ribbon, afterall), and you need to have a pretty quiet pre to pull up the gain. The 421 can take a gunshot of SPL (just like their ads used to show), handles all sorts of sources (especially bass instruments), have that 5-step roll-off switch to tailor the LF response, and are overall a lot tougher to kill off.
The only thing that I really disliked about the M260 (and the M500) is the fact that when they died (or were stolen), it would break my heart. When a 421 crapped out (actually, they would just sound real funky, nasally), I'd just shrug my shoulders and mutter at the drummer who slammed it.
So, if you are NOT slamming the mic and can baby the thing, go with the 260, you'll love it. Just have some Kleenex handy when it dies!

Beyer 260 I like on trumpet, guitar amp, violin. They are sweet with sources that have harsh or scratchy highs.
I had mine rebuilt by Beyer at a location on the East coast, don't remember where. They came back sounding like new.
Store them vertically to keep the ribbon in the proper shape.