I chose the Bronica ETR system last year, and think it's great. I was considering the Mamiya as well, but I preferred the sample images I could see from the Bronica. I can't say if one was sharper than the other (with web resolution images) but I preferred the bokeh from the Bronica.
Now that I have had the camera for a while, I can report that it is very well built, works great, and delivers beautiful images. It is also a very complete system with many options for lenses, finders, winders, etc. all of which are very cheap these days.
I will also weigh in on the lenses available for the Bronica. I have a nice set of MC (original) lenses. I am pleased with the results they give. However, the universal opinion is that the newer PE series lenses are sharper and more flare resistant. So it's your choice, because obviously the MC lenses are cheaper.