I use a Bronica SQAi and a Mamiya RZ67 II.

For normal portraits I much prefer the square format. It just seems to suit portraits better, especially head shots and head and shoulder shots etc. However if you want to do a full length or even 3/4 length portrait the 6x7 format can fit better.

Other main difference between these two cameras is the Mamiya is not all that comfortable to use hand held, it just doesn't feel natural. I am right handed so I end up supporting the beast with my left hand and using my right hand for the focusing ( seems to feel better then the opposite ), so a tripod is almost a must. The Bronica on the other hand is no more cumbersome than a large 35mm SLR, held with my right hand and focused with my left, like a 35mm SLR.

There is a hand grip for the RZ and that may improve things but I have yet to try one.

All in all for Portraits I would be 90% in favour of a square format.