Straight line performance is close enough to no difference. M3 is the sharper drive, though my seat time in the AMV8 is limited, all reviews seems to back this up.
I think you'd be very pleased going from the E93 to the AMV8 roadster. Both are very nice cars but in convertible form I'd find it hard not to pick the Aston.
'10 Land Cruiser
-prev M-cars: E60, E92, E46, E36