Keep us posted with what you decide. I am in the same pickle. I am looking for a 550i non M-Sport bumper as I like the looks better. I may even wait for the LCI bumpers to come out and snag one of those. I personally don't think it's a big deal to have a M-Sport rear bumper and a standard front bumper. They both have the same lines and distance from the ground. I measured with a buddies F10 parked side by side.
2011 535i Sport (Prem 1, 2)