I can't offer any experience with the mk2/2s/2h as I haven't had a chance to run them. However, I do have some really nice omni tapes. That said, I counldn't be happier with the mk21's. They have given me all the natural full sound that I expect from an omni, but they are forgiving enough to run indoors fob. Onstage is where I think they truly shine. I have only compared them to onstage mk4 sources, but its pretty remarkable how much presence they can offer. I discussed the mk21's with Peter Costello before I bought mine. He does sound work for many bands and has the opportunity to put mics onstage for the likes of RWTC and others. He mentioned that he had compared onstage results with tlm170s on many occasions and he was so convinced, he didn't even bother with the neumanns anymore. Granted, this is all a very personal opinion, but it still meant alot to me.
I have only made one really bad tape with the 21's... it was indoors in an old theatre, way fob, for a Mule show. Turned out entirely too bass heavy. If I really cared, I could have done some work in the post to fix it, but there were a few other card sources that sounded really nice, so there just wasn't a need.
The other reason I really like the 21's is b/c they are very convenient and easy to use if you have a NOS kwonbar. No problem clamping that arrangement on any stand and its still semi-low pro. Convenience means a whole bunch to me these days!
FYI: there was a guy looking for a pair of mk4's to go with his cmc6's. If you jump on it, you may be able to hook him up. You still have some time to get on it before the price increase. If you wait, it'll cost you at least $130