I've used the old style Hasselblad 45 degree prisms for a long time. My preference is the 45 degree instead of the 90 degree.
If you aren't aware of the difference the 45 is at an angle where your face/eye is at a higher level than with the 90 degree. With the 90 degree the top of the camera is level with your eye.
The other thing to be aware of is that some come with a light meter and some don't. Mine didn't because I always used a hand held light meter.
Another thing that changed during the years is the shape if the design. Some of the later ones, I believe, were shaped so that you could attach a polaroid back to the camera. On mine I had to remove the prism and use the "stock" viewer whenever I wanted to attach the polaroid back.
All that being said, if you don't have a polaroid back, and don't care about a light meter built in, any of the old ones are just as functional as the newer ones.
However, with the amount of cheap Hasselblad stuff floating around, I'd try to find the newest and best stuff available.
Michael