(Note: I don't have an M6... yet /ubbthreads/images/graemlins/wink.gif)
I would prefer the regular, non-crenelated bezel just because I personally wouldn't use it as weapon and I just like the standard look better.
HOWEVER, given the M6's power, I would say that the crenated bezel, in allowing you to see that it's ON if it's left bezel down, is a safety feature and not a bad idea!