Hi,
The non-metered finder is probably a little bit lighter, but honestly, the whole camera is pretty lightweight anyhow, so I don't see having a problem with weight when using it. It's a medium format SLR; you pay for the advantages of the larger frame and SLR design by taking on certain other disadvantages. If an SLR is still too heavy for you, you can get a rangefinder. Of course, then you lose all the advantages of an SLR, and unlike a minor weight issue, these are advantages that can seriously affect what you get in your pictures and how you take them.
I don't think the prism used will make a huge difference. The main advantage of the non-metered finder will not be lighter weight; it will be not having all those meter thingies sticking into the frame from the side of the viewfinder. If you want it to be as lightweight as possible, use the WLF, though, as mentioned, shooting verticals is horrible with it, even on a tripod.
Also, you may have trouble finding a non-metered prism without the common mirror separation line. I sold my metered prism thinking that I could easily use the money to find a non-desirable non-metered prism, and still have some hamburger money left over. I tried, and failed. When I have found them without the line, they are being sold for dumb prices, e.g. as much as I spent for the entire camera kit in the first place.
I decided that I am just going to move to a Pro TL eventually anyhow, so I might as well forget the search for the old-model non-metered finder. I love the feel, construction, and styling of the older cameras by far. But I can't argue with exchangeable magazines and a non-metered prism that is not prone to problems like the older ones are.