How did your shots turn out, Galah?
I have the Pentax 2x straight magnifier for the 6x7. You take off the eyepiece and screw it back in through a hinged flange, so you can flip it up. It allows very precise focusing but pretty much requires a tripod and slows the shooting down quite a lot. Installing and removing it also is slow and there's no way to safely store the camera with it installed. I recently got a split-prism screen installed and am looking forward to giving it a shot. I'm guessing the wides are going to be difficult to focus, but the 105 f/2.4 and the 150 f/2.8 seem to generate quite a bit of offset for the prism. For what it's worth, with a 75mm f/4.5 or a 55mm f/4 the prism doesn't darken until you get inside under poor lighting.
On the whole prism debate, my two cents: the 6x7's TTL meter is ancient 60s technology. It's averaging, which isn't a problem as long as you are aware of what you are doing with it. It does draw a lot of current and will be the biggest drain on your batteries. There's apparently a version with a momentary switch (turns on for 10s when you push the switch) and one with a plain mechanical switch. I have the plain switch, and it's very easy to forget and leave the meter going. This will drain your batteries in ~24h. A spotmeter will let you do the zone system, it will let you use the lighter unmetered prism, and you only have to buy it once to use it with all your cameras. It's not as handy and you don't get the confidence-booster of seeing the little centered needle, though.
Also, I am a proponent of the grip. I want to do some tests to be sure it's not affecting the sharpness, but if nothing else it is much easier to carry around. Get a big wide hippie camera strap too.