I went from Mamiya 7 to Hasselblad, but I think for you, Mamiya is the choice. You're talking about low light and flash, which I think the Mamiya is *a lot* better for. I assume you're talking handheld, not tripod, so, for me it's Mamiya every time.
If you're shooting on a tripod, then I think Hasselblad is probably nicer. The ground glass works well on a tripod, and of course there is the accuracy of the SLR vs. range finder. Many Hasselblads have mirror lock-up, so camera shake is not a worry. For hand held use, which it sounds like you're after, Mamiya all the way.
If you're happy with a fixed lens though, check out GF670 from Fujifilm, it's probably a tad cheaper than a Mamiya 7II with lens, and folds up to be very portable. It's a beautiful camera with a fantastic lens.