Danny, I think I've gone for gloss every time so far and it seems to scan ok. I'm waiting on a friend returning the FP-3000B I shot at his wedding to find out how well that scans.
The image area scans ok, but the tacky bit round the edges can be a bit untidy.
I'm not entirely sure about the RZ, but with the RB Polaroid backs I've now bought two and they're different models. One came with the P-Adapter and one doesn't need it. I think there are a total of four different Mamiya Polaroid backs, plus a couple of third-party models - the Swiss Arca one looks good, it somehow lets you take two side-by-side images that fill the image area without the heavy black bars.
I haven't tried a prism finder, the WLF is awkward at first, but you soon get used to it. The weight penalty for carrying a prism seems not worth it to me.