I’ve run my 21s next to Noah’s 22s a handful of times. DSatz’s assessment is fairly accurate - the 22 in practice seems to give slightly more defined imaging of sources, and the 21 gives a more “realistic/hi-fi” capture of the room, which can give a “hazier” image.
My thoughts for taping: The 21’s bass response is noticeably better than the 22’s to my ear. The 21 reaches deeper and maintains its definition to lower frequencies. Though this isn’t to say the 22 lacks bass by any means. I also prefer the 22’s treble to the 21’s: I hear a softer and more pleasant mild roll off in the top end of the 22, whereas the 21’s treble is aggressively honest and at times unpleasant. This said, I’ve preferred my results on the 21 when massively cutting the angle to much lower than we as tapers are used to using - the best results I’ve gotten from it are in A-B, followed by 30cm/60 degrees. I think the 21 is a LOT more open than we give it credit for. To wit, a good comparison I think would be to think of the 21 more as an omni you can aim, whereas the 22 is more of a cardioid that brings in the bass. The 21 definitely captures way more room and the 22 captures less room, when deployed similarly. From OTS, I think the 22 is a more forgiving cap and will be more likely to pull a tape that is more enjoyable to listen to, though there’s something to be said about “getting it right” with the 21 (which is still something I don’t feel I have figured out just yet).
It isn’t a 1-1 mapping, but on 12/31/18 Noah and I both ran out mics on the same stand 15’ up in the air. I believe Noah ran his mk22 pair at 50ish cm PAS, and I ran my 21s in 35 cm/65 degrees. We both used Mixpre-6’s, Nbob KCYs, and Naiant PFAs (though Noah’s run 60V and mine 48V). This should give you a better idea of their similarities and differences. If the etree links are dead just poke us and we would both be happy to get the tapes in your hand so you can spin for yourself.
In the studio, I only have experience with the 21. I don’t experience much proximity effect on guitars or drums, which is mostly what I use it on. Certainly nowhere near what I get with the mk4. I like this a lot actually, as I can throw a cmc521 on my guitar cabs and get an honest omni-like capture of my Mesas while getting *some* rear rejection of me playing in front of my amp. The treble is also a lot more manageable on a source up close, ime.