The OP has Hasselblad film backs. If he goes with the Mamiya he will have two types of film backs and I can tell him that is a real pain in the butt.
The Mamiya 645 is a good budget system choice for the format. Strong sales means there's a reservoir of decent quality bodies, lenses and accessories at good prices. The only must-have accessory for me was a motor grip that greatly improves the iffy ergonomics of the body. But don't you already have a 645 system, albeit a rangefinder? A contrarian option is to go up to 6x7 and to look into the Mamiya RB67--big but remarkably versatile with good lenses. Got one+a couple of lenses last summer and can't stop shooting it. Good thing my Mamiya 645 and Bronica SQ-B kits aren't the jealous kind.
In this case I completely agree, though: Brian already owns a Hasselblad SWC which is a more substantial start into the system as may be apparant at first sight.
Well, I can only pass along my experience. I started with a Mamiya C220. It was OK. I then tried a Pentax 645. Lenses were fantastic and the price was right. I hated the camera. Finally, bought a Hasselblad and realized that I loved the square and loved the ergonomics of the camera. If you like the sqaure, get the Hasselblad if you can swing it or a Bronica if you can't. For me, the format is more important than anything else.
Matt, the reason I don't have a wide angle lens in my group is because I have an SWC, and also a 50mm for the Mamiya 6, and a 45mm for the RF645. So I've certainly got the wide angle lenses covered. The main thing I'm looking for with either system is telephoto lenses.
GWC, I do already have the 6x4.5 format in the Bronica rangefinder, but I only have two lenses, a 65 and a 45. I feel there's no too many options there besides use as a hand-held or hiking camera, which it is great for! 6x7 is not a format for me. I've tried it in the past and don't particularly care for it.
Mark, I agree with you. I love the 6x6 format. Square is great, and I think I'm leaning more towards the blad because I can get 6x6 and 6x4.5 all in one camera, whereas if I went the Mamiya route, to go square I would lose a lot of negative.
I'm still thinking. But, like I said, I'm really leaning towards the Hasselblad simply because I already have backs and because of being able to shoot square and 6x4.5 at the same time, and not having to turn the camera on a tripod (sounds simply, but is really annoying after a while).
Thanks everyone for the suggestions so far!
"The other question is reliability. I would imagine the Hasselblad system would stand up to use better than the Mamiya 645. How about the electronics in the newer 645 models? And problems?"
If new, the Hasselblad would be on top. However, used, it may have more than its share of wear and tear. Of course, the same may be said of other cameras, however, more likely to have been in amateur use and have better care.
If both were equal, it seems the Hasselblad would make more sense and is what your preference is anyway.