I use both range finder and TLR at the moment, and used to use Hasselblad. For accurate, close up focusing. I much, much prefer range finder focusing. I find it so much easier than either relying on my eyes to see focus, or even the split screen, which are good, but I do struggle a bit with them.
For tripod work, I much prefer ground glass/SLR, not just for looking through the lens, but the convenience of looking down on the screen, not crouching to look through the finder.
So for me, carrying it around? Mamiya every time. But if I'm using a tripod 99% of the time, I'd take the Hasselblad. I will say that often range finders have more accurate framing than you'd think, and unless you *really* want to include details at the edge of the frame, accuracy may not be that important.
It's a personal choice, but I find I like both for different things, and RF and SLR each have their place.
I have the Mamiya 7 and I love it. There's a lot of plastic in it so it picks up scratches and general battering marks really easily. It's my workhorse camera. I mostly use the 43mm lens (which is not available on the 6). I am going on one of Bruce's workshop courses in April which I am very excited about. I will use my Mamiya 7 & 43mm and my Hasselblad & 50mm so that I can do both BW and colour.
I have used my Mamiya 6 for traveling - usually with all three lenses. If I really want to travel light I have a little Fuji GS645s. The Mamiya 6 actually fills the same bag I used for a C330 with comparable (55, 80 or 105, 180) lens set. A little lighter, though.
I like RF cameras when I am in unfamiliar and busy places. Better general awareness. I prefer to compose on a camera screen, but it does not seem to make much difference to the final results.
I have both Mamiya 6 with 3 lenses and a Hasselblad. The Mamiya is much easier to focus, especially in near-dark conditions. The issues of using polarizers or grads are non-issues to me--when mounted on an SLR the grad line is impossible to see anyway (even with the lens stopped down). I mark the dividing line on the outside of the filter and just estimate it. You can just bracket for precise position also.
The issue of rangefinder viewing and accuracy are very overblown--I have never been bothered by the final image. Perhaps you OCD types who insist on always printing the whole frame will care--in which case it's time for your meds. I have not found the Mamiya 6 to be fragile, and take it hiking regularly. I believe the reason for winder breakage is the fact that some rolls by some manufactures are spooled on very tightly and won't come off the reel at the film end. In that case, just don't force it. Anyway, I very much doubt the winders break with no reason. It is true, though, that there are no spare parts, and even bargain bodies now are going for near $1000. It is much riskier than a Hasselblad in that case. The Mamiya 50 is simply the best lens I've ever used. Period.