They really are completely different capsules... The mk21, as a subcard, has a very natural sound compared to a card. It also has significantly more bass extension than the mk4 variants. Compared to an omni, the reward rejection is very helpful.
They are both outstanding... I originally went with mk21 + mk41, thinking I could get by without the mk4. And then I had to have mk4's. And sold my mk41's... I just don't run hypers often and also have mg200's and 210's. I tend to run close, where I just don't use hypers. Though I have run both of those outstanding hypers stage lip with great results.
I do not feel that mk21's are all that suitable for headworn situations. I don't believe that config can provide the capsule separation distance sufficient for great imaging. They're very good in that app, I just much prefer the mk4 or mkv4.
Given a choice, I'd take mk4v's over mk4's for the slight high freq bump. And they can fit better in low profile applications.
Stage lip and on stage, the mk21's are really incredible. 10 to 15 rows back... indoors I'd usually run mk4's. Outdoors, mk21's... No card or hyper will compare to the mk21 outdoors stage lip or on stage. Not even close.
Tough call!