I'll be the heretic and say I think both are retarded. That said, I voted 10mm -- because, if you're going to be retarded, one may as well go full retard.
But, as a caveat .. I stick to guns in 9mm, 5.56, and 7.62 -- all NATO, all the time.. because I shoot. A lot.. and I really don't need to be paying a premium to do it, and I don't have the free time to hand-load the amount that I'd shoot. Over the course of 6 months or less, the $$ difference between 9mm and 10mm would equal up to a whole new gun to play with.
That said.. a Mark 23 may be the gun that makes me add a new caliber. And from there? I'm probably back in the UMP game.
Damn you guys.
I really should just give up on all y'all and just go start an FN collection.