I'm not very experienced in the medium format category, but this fall I did buy a Hasselblad 503CW with the 80 CF T*got it relatively cheap too from a working photographer.
( I also bought a 160 CB lens, brand new, but got it very cheap, a very nice underrated lens btw)
I also bought a Mamiya RZ67 Pro II about the same time, that I got really cheap, with two lenses, cost me about 1/5'th of the Hasselblad, but I think I was rather lucky with that one ^^.
Now, I have used 35mm a lot, especially the Canon 1v, which can use all the lenses that my digital 1ds mk II can.
And I agree with agfarapid, shooting 35mm vs shooting medium format with the 503 or the RZII is a huge difference. Currently I have not yet warmed up to the Hasselblad , I find it difficult to compose properly (due to the square format) and I also struggle a bit with focus, as I have some kind of accu-matte split screen thingy, I constantly focus/recompose while shooting models, but recomposing is very confusing due to the flipped screen you are looking at. (left movement goes right and right movement goes left, the same with tilting left or right).
IE. until you get the hang of it, you will have serious issues tracking a small child running around, that's for sure
I find the RZ pro II a bit better concerning focus, because it has a pure acu-matte screen, no split screen, it also focuses trough movement of bellows and I find it much easier to adjust than on-the-lens focusing (always turning the wrong way :P ).
That said though, I do not regret buying any of my medium format cameras, just looking at the negatives is a jaw-dropping experience, 6*6 or 6*7 isn't a huge difference, really, they are both dang huge Still, for christmas this year, I shot 35mm only, to capture my nephews and niece as they were running around, playing, opening gifts etc.
If I were you, I would use a Nikon 35mm for action/snaps and the medium format for controlled environments/stationary people/sleeping child/landscapes etc, at least until the Hasselblad experience is second nature to you.