Micheal, looks like your too far along to change this now but I think mounting M3ow on the ceiling would be a lot less work and sound better. I've listened to both these speakers and I believe the M3ow are almost as good as an M3 while the M3c are a bit of a compromise. Another observations between the M3c and a FMB mounted M3ow is that the on-walls are more amiable. The M3c's tweeter only has a few degrees of aim before it hits the grill. Further, In theory, you want your all the speakers to image well with each other so that objects (sounds) can be placed exactly in 3d space. The on-walls will do that better the the in-ceilings.

Now, I have not heard either of these guys in an ATMOS setup and suspect that when you in the middle of a movie I doubt the differences will be extreme. Bottom line ... bet both setups will sound awesome.