Since we are posting on an older thread, I'll add in my 2 cents...
I love the exhaust sound on the 50i, to me, probably the best sounding BMW exhaust out there (and yes, superior to the M). I also love the fact that I can get Active Steering and Vermillion Red on the 50i.
However, if I could only have one car and I needed the ride height of the X, then I would think the M would do just fine. I do however still need to drive my friends 50i with Dinan tune....
PS - as posted only a test drive can tell you want you want to know.