The two systems are both of excellent quality, so it is a happy choice you have for yourself.
Do you use a lot of fill flash in high pressure circumstances (e.g. weddings)? If you do, you should probably make your comparisons using the leaf shutter lenses that Mamiya offers, rather than their more usual versions.
And if you do, you should probably factor in the cost for the appropriate Mamiya power winder, and special connecting cables.
If high pressure fill flash isn't important, I'd suggest handling both systems, if that is at all possible. Their ergonomics are very different. Personally, I find my Mamiya cameras and lenses much better suited to me than the Hassleblads I used to sell.
As for the square vs. rectangle issue, my judgment is probably affected by a couple of factors:
1) I have Mamiya TLRs to satisfy any need I have for square format, as well as leaf shutters and easy close focus;
2) I have a flip bracket that makes tripod use and switching to portrait orientation quite straightforward; and
3) I really like the Mamiya left hand bracket.
If I were you, I'd also check things like close focus distance for the lenses you are interested in. You can still get the info for the manual focus Mamiya lenses from the Mamiya UK website.