From a drivers perspective, I like the idea of the M3 better. Its a more driver focused vehicle. MB still has a long way to go to get the C class AMG to be on par from a total package perspective with the M3.
Whether or not BMW delivers with the newest iteration is a different question entirely. Based on their previous cars I have little reason to doubt them.
I've chosen cars before simply on the basis of engine, and frankly I was disappointed. As much as I loved my 03 Corvette Z06, I could have had an E46 M3 for the same price and after driving both extensively I know which one I would pick going back in time.
Best of luck with your decision!