I own and use both. I think they both do a good job of being a higher powered version of the respective greenback they are representing, 12M's for the M65 & H30's for H75. They are great mixed together or an M65 mixed with a V30 is also very cool.
The only issue I've had is when I used a pair of M65's together in a Bogner 2x12, I thought the bottom end was kinda loose and not really suited to heavier styles. Mixing in an H75 or a V30 with the M65 fixed the problem. An H75 with a V30 was too much for me in the upper mids / high end.
I've voiced the opinion before on here that I thought the 65's were too loose for metal, but others have dis-agreed, and I believe it was Jer-Evil who posted a vid using the 65's that sounded pretty good. For me, 2x65's in the Bogner OS with a Twin Jet or a 5150 was a mess. Not good at all.
If I was loading a 4x12, I'd probably go with a pair of each type of Creamback, but a 4x12 with 2x65's and 2xV30's would give you a similar vibe to a Friedman cab with higher power handling. I think either one would sound fantastic, but I would definitely shy away from using 4x M65's, especially if you already have a greenback cab.