I owned the M 80 for a few years, and I actually use the B7K.
The M 80 is a very nice, good sounding and versatile pedal. The preamp and DI parts of the pedal are really nice, and i used it a lot for small gigs. The OD part is quite good too, but i disliked the fact that the "color" switch automatically comes in when you activate it.
I bought the B7K for the same purpose, but mainly for the OD part. The thing that made me choose the B7K was the bright, modern OD sound that you could get out of it. You can still get this sound with the M80, but the B7K (or B3K) sounded really better to my ear.
If you're looking for a good OD, i would suggest you to go for the B3K. If you want a versatile tool, you should go for the M 80. If you have enough money, you should go for the B7K.