The M157 of course benefits from already having forced induction from the factory. It is a smaller motor though that revs to a lower 6500 rpm. The M156 as a virtue of starting in naturally aspirated form has the better flowing heads and the better valvetrain and cams for top end breathing. A little bit of boost goes a long way on a motor that starts with high volumetric efficiency.
The C63 features the Weistec Stage II blower which means a 2.3 liter unit and not the larger 3.0. The video unfortunately does not offer the greatest perspective but it is fairly clear to see the C63 pulls the E63 up top. Impressive considering this Renntech tuned E63 has been beating up on all kinds of fast cars. The C63 of course is lighter (not by much) but it shows what the platform is capable of.
Tuning an M156 motor is more expensive but ultimately this is the superior power platform. Why? Larger motor, higher revving, and no direct injection fuel limitations. It does take quite a bit more money but considering cars with the motor are now cheaper to acquire with an M156 than those with an M157 value is currently in the M156's favor unless one goes by tune only performance.
Regardless, these are two impressive AMG engines that are only going to get better as the tuning for them continues to evolve.