I decided not to wait for the MP9 and am getting a MK9 instead
Good choice, You won't be dissaspointed.
The stainless MK-9 is a fantastic mini 9, Mine rides daily very comfortably in a kramer pocket holster.
It is time tested, super accurate, mine is 100% reliable, very strong and has great ergonomics. The polimer Mk will also be a fantastic gun and certanly lighter prolly about 6oz, I would get one if they were avaiable in my state,( They won't be) But I would still keep my stainless.
To put it in perspective, if you have access hold an empty kel-tec p-32 and that is about the difference in weight between the stainless and polimer MK'S.