Well I can vouch for both the Bronica Etrs and Mamiya 645 systems
and would recommend the mamiya over the bronica.
physically the mamiya body seems more robust and the bronica body felt
cheap.
i didnt like the idea of the lenses having shutters because I have read elswhere
that those seiko shutters can give trouble.
i also didnt fancy having to get my lenses
serviced every so often.
I am currently using the Mamiya 645 Super, this was the first 645 body from the
second generation family. all the lenses from the previous generation will work and the
all the interchangeable backs, prisms and grips from the later Pro and ProTL models
can also be used. Most of the standard prime lenses in the range take 58mm filters
which is really nice and handy.
A lot of people skip the 645 format and go instead for the 6x7 larger format but I find
the hardware prices a lot higher for the larger gear.
The really nice thing about the medium format SLR gear is that they are modular systems.
To start off with all you really need is a body, film back, lens, winder and prism and as
you get used to it you can be on the look out for extra bits like, 120, 220, 135 and polaroid backs,
grips, lenses, screens, prisms and cable releases.
Personally I would go for the Mamiya over the pentax and bronica! I remember doing a lot
of research on the 645 systems before I bought into them and I figured the Mamiya was the
way to go and while the Pentax has KILLER optics the mamiya I felt was the most modular.
Here is a handy resource for the Mamiya >
http://www.photo-phil.org/Mamiya.pdfI hope this helps.