I really have limited experience with a variety of medium format cameras. I bought a Mamiya C330 in 1974 (I think) and Mamiya TLR's were the only MF cameras I've really ever used until last year. I bought a used Pentax 645 at that time.
Mamiya TLR's are solid as bricks, very dependable and simple. The lenses are good and there is a ton of bodies and lenses out there on the used market at bargain prices. The bellows focusing means close-ups are easy to do as long as you remember to take into consideration the exposure compensation needed. What I never liked about them is that they feel top-heavy and off-balance to me when handheld. I've always considered them to be tripod cameras. I've been told that adding a prism makes them more handholdable but I've never tried it.
I fell in love with the concept of the Pentax 645 when it was first introduced in 1984 (I think). The format size is essentially what I get from cropping my 6x6 negatives and there are three more frames available per roll. The camera is made for easy handholding with a built-in motor drive, grip, prism and it has great ergonomics. The mirror dampening is outstanding and it's pretty quiet for a MF SLR with an instant-return mirror. The lenses are excellent and, since they don't use shutters, they cost less. The downside for the Pentax 645 would be if a photographer needs interchangeable backs and finders and, especially, if there is the need to use a lot of balanced flash (it has a cloth focal plane shutter). I don't even own a flash so it's not a factor to me.
I finally bought a used Pentax 645 20 years after first seeing one. I got the original manual focus model with 35, 45, 75 and 150 lenses. The used prices for the equipment was something I couldn't pass up. It's become my most used outfit.
Of course I've always wanted a Hasselblad too.
