How much over MSRP would that used one be?
Normally I would say wait for a new one, I always have in pervious cars, but I had this same dilemma 2 months ago with my M4. I wanted the car now though and did not want to wait. That said the used one I got was almost exactly what I wanted other than color and was not above MSRP. So I pulled the trigger on the used car, and have been loving the last two months rather than waiting for a new zero mile car. Also keep in mind that no matter what anyone says now about any dealer there is always a chance they add a mark-up, even "in writing" means nothing. Anything you get in writing now is not a legally binding contract for when the car shows up.
Life is short, get what you really want though. You have a car you love now so wait for the new one if that will make you happy, or get the one one "in front of you" now and enjoy not waiting... you also will not need to wait for the break-in time.
