I too had both the NC-2, and the HC-1 (one of the original 90 degree prisms). Traded them for the PM5 and PM90. As one grows older, eyesight gets worse, and the newer prisms (PM5 ~ PM90) allow for super easy diopter changes. Just screw in a new diopter eyepiece and no more fuzzy pictures!
(Something to think about years from now!). With the older NC-2, diopter change is possible but it is a pain, as the the diopter for the older NC-2 is just the glass piece and you have to take apart the original eyepiece. For the HC-1, you will most likely have to find some optical lab to grind a diopter for you... no joke... as the diopter size is beyond standard size!:rolleyes:
I find the PM5 (45 degree) much brighter and it has a higher degree of manigification then the PM90, so unless I am shooting 645 (which I do many times), I use the PM5.
Also, if you do intend to shoot 645, and you plan on shooting both horizontal and vertical format, trust me, you will need a 90 degree prism, unless you enjoy looking strange as you try to turn the camera body to capture the image. The A16 back is formatted for 645(horizontly) and in order to get it vertical you are going to spend half your time turning and twisting to get the shot!
Hope this helps