I've used the various Mamiya 645 models, the older Pentax 645, and Bronica ETRSi and currently use a Bronica SQ-Ai. I don't think there is a lot of difference in weight between the two Bronica models. The reason I bring it up is that you could just add a prism, a Speed Grip, and a 120-J 6x4.5cm back to your existing SQ-Ai. The Speed Grip, if you haven't seen it, adds a manual winder in the same position as a 35mm SLR handgrip. Presto, then you've basically got a camera that looks, feels, and acts like a 645, but with the option of switching back to 6x6 anytime you like. Sell your 35mm gear, and put the money into your existing SQ system, that's my advice.