I think you may be rationalizing your decision.. I have had both as well and find the M ride to be better over bad roads than the non M suspension on the 50i with adaptive.. Maybe its me but have talked with others familiar with both and they feel the M can be more compliant. I do envy the 8 speed a bit but really only for mileage. Short of that I wouldn't change a thing outside some better build quility, My '10 M seemed put together a little better than my '13.
Keep us posted on how you compare the two after having your new one.

__________________
2013 BMW X5 M