I had a ETRS and I have a 501CM. I also had a SQB, which is the Bronica 6x6 camera.
For starters, the main question is do you want 645 or 6x6? This is not a trivial issue, it is a very different look the square vs the rectangular. There is also the practical issue of getting 15 shots per roll vs 12.
I'll start with the 501CM. It is a wonderful camera, I find it comfortable to hold, very nice to operate and the results are superb. Prints and scans come out really nice. The camera is relatively heavy, if you never used medium format gear before and if you add a prism it can be a bit of a shock at first.
Regardind prices, don't just think about the body and standard lens, have a look at the other lenses and accessories. I got a Hasselblad 90 degree prism for a bit more than what I sold my ETRS kit. A 50/75/150 ETRSi kit with the newest lenses plus prism plus grip and a few backs will cost you less than £1000. A 501CM kit with 50/80/150 plus prism and backs will cost you £2000-3500 depending how old/new lenses you buy.
The ETRS is lighter and fitted with a 90 degree prism and a speed grip it handles very much like a 35mm/digital SLR. It is fast to operate and easy. The minus points are that comapred to a Hasselblad it feels like a number of bits together. What I mean is that a medium format camera is indeed a number of bits togehter, back, body, lens, finder but when you hold the 501CM it feels like a solid block. The ETRS is like bits put together with bit rattling here or there, the prism jiggling a bit if pushed, the knobs and dials all a bit loose. There is a reason why it is cheaper, well this is it. This also goes for the SQB I had. The design of the ETRS/SQB is also a bit less thought out, things stick out and protrude here and there, the Hasselblad has everything flushed and slick.
Image quality wise, well the Hasselblad is sharper but the Bronica is sharp enough. The main difference I see is that the Hasselblad prints I have are with more contrast and better overall colour but if I was stuck with Bronica I wouldn't be unhappy.
So, basically, the differences are that moneywise, everything with Hasselblad on it is anything from 2-5x more expensive, you get better colour and contrast with the 501CM and you also get a heavier camera. And there is the obvious format difference. And if you want digital, as already said, there are a number of options for the 501CM but none for the Bronica.