Ok, since this has gone unanswered I'll take a stab at it but I'm no authority on the subject. The Mamiya 6 is an older design and people who use it seem to love it. There have been reported issues with the film transport mechanism in the 6 and from what I have learned there is a discrepancy as to whether this can be repaired, some report no parts available and I've seen another post that said parts are still available.
I recently picked up a Mamiya 7 with the 80mm lens from a forum member here-thanks again Ryan
I picked a 7 since it can do 6x7, I have a Minolta Autocord if I want to do 6x6. I was considering a 500CM or a Rollei TLR but I have the Autocord and if I want a 'standard' rectangular print size my 6x6 becomes a 6x4.5 while the 6x7 requires little if any cropping.
Some people have a hard time composing with the 6x6 format. I really like the 6x6 format but nearly all my compositions taken with the Autocord only work in the square format, if I try to crop to fit an 8x10 for instance the composition falls apart. With all the formats I use I tend to compose depending on what I see in the viewfinder or on the ground-glass. I haven't been able to train my eye to compose one aspect ratio within a different format. My 35mm stuff, tlr and my 4x5 and 6x7 usually work best in the aspect ratio of the camera they were composed in which can be frustrating if I want to use precut mats.