I say just keep the 2011. It new, reliable and will have a bit more room in the back. The P is a nice car but if you need to get ride of one I would keep the M. Just my 2 cents. Good luck.
08 BMW X5 3.0si Space Grey
14 Lexus GX460
11 BMW M3 JZB ZCP 6MT, 02 BMW M3 Conv 6MT & 02 BMW M3 Coupe 6MT