I am not a pro + have never used either mic, but the Green Day album has C12's on the toms, and they sound damn good...
I wish I had a C12. That's a very nice vintage tube mic. You never know, you might like a vintage rock sound better than your normal Neumann, but you didn't say what style of music you're recording...
My philo. is that if you're not at the tier you want to be in the end, then what could it possibly do to try different (even crazy) new equipment. If you're making tons of new fans with the U67, and it makes you smile and sleep better, then I'd wait, but if it's just good, then, who cares.