If going for a prism, get a 45 degree one. Much more comfortable than the 90 degree ones.
The NC-2 is a perfect little prism. The later ones are equally good.

90 degree prisms are only needed when using a 6x4.5 back and you want to turn the camera on its side.
When not doing that, what a 90 degree prism does is force you to hold the camera higher, in front of your face. Which also moves your arms out, away from your body, and produces a rather awkward, less stable, stance.