6Cm is going to have higher velocities than a 6BR.
If it is for shooting or varmint hunting where barrel life is a concern, the 6BR is a better choice.
If it is for small/varmints/medium game like deer/pronghorn/coyotes, the 6CM makes sense.
Don't leave out the 6GT, 6x47L, 6XC if you are exploring midsized 6mm cases. There are several others too, that are a bit more work but produce amazing results. 6PPC, 6 Dasher, 6BRA, etc.