When I had a 680, I personally liked the type I body as the plastic focus knob felt sturdier and was easily to grip with my big fingers. The type III body added a spongy grip to the knob which after extensive usage would slip and require extra finger pressure to engage the knob.
IIRC the prism is really a reflex finder, basically a mirror in a box, so it is fairly dim and not corrected L to R like a true prism.
The type III battery holder is more readily available and can be modified to work on the type I and II bodies.
If you get an extended WA bellows and the 40mm ext. rails and leave them on the camera, it makes it easier to use esp. with WA lenses.
Come by the shop and we can talk more.