I've got prism viewfinder and waist-level viewfinder with loupe* for Mamiya 67 RB, and I can tell you I have used prism one only once. There are two reasons: the prism itself weights almost 1 kilogram (2 pounds), and it has no metering. As per portrait mode, Mamiya has got rotating back, so it's not a problem. Left and right are reversed in waist-level viewfinder, what is problematic to some extent. For me prism viewfinder (with exception of the one from Mamiya

is more convenient as I don't have to look at the camera from above, what is rather unnatural in most cases. The loupe, on the other hand, makes precise focusing easier.
* basically collapsible chimney loupe