If you look a the F87 M2 N55B30T0 power curve it just is all low end and the torque quickly drops off. How does that make sense for an M car designed for track use?
The S55 isn't exactly BMW's best top end design but it offers a much broader curve with stronger pull toward redline. It's a better motor.
So what is fueling these S55 into the M2 engine rumors? Spy photos from CAR showing cooling changes that look similar to the S55 cooling system. Not to mention Nurburgring testing from last year of an M2 said to have the S55 under the hood.
It's possible only the CS model will get the S55 motor and the standard M2 will stick with the N55B30T0. If that is the case, the CS will be a much better buy.
We'll know soon enough.