I went with the Hasselblad, simply because the price of used ones were down where I could finally afford it. I got a 500c/m + A12 back + 80CF for less than I paid for my Nikon D70. And I would do it again.
That said, I had looked long and hard at the RB/RZ-67. I liked the larger negative and the rotating back. But it was heavier, and I needed to get my Hasselblad first. For me the Hasselblad is "good enough" for what I want. If I want a larger negative than 6x6, I would skip 6x7 and go to a 6x9 in MF or 4x5 in LF.
As for the square vs. rectangle format. For me that is a non-issue. The 6x6 negative is large enough to crop w/o noticable (to me) degradation in the image quality. Then any MF would have that effect, since I came up from 35mm, with its even smaller negative.