Rickruss, I'll bet you end up with all three in fairly short order! They ALL have very useful fields of view. It's useful (and fairly accurate) to consider the equivalent fields of view in the 35mm format as having half the focal length of 6x7...
The 50mm Mamiya corresponds to a 25mm on my Minolta CLE; this Voigtlander lens was my most used one on a recent trip to Spain and Morocco, especially in narrow urban streets.
The 80mm is like the slightly wide normal 40mm f/2 on my CLE. This 40 seems to be perfect for most shooting, a remarkably useful field of view. And the slightly wider 35 Summicron has been my most-used Leica lens for many years, as I understand it is also for other users. In recognition of this I just got a 75mm for my 6x7 camera. I think you'd love the 80.
And the Mamiya 150mm gives about the same look as a 75mm for the small format. I don't have this focal length in 35mm, but I like 85mm in my SLR, though 90 seems too long on the Minolta and Leica. I'm tempted to get a 75 f/2.5 Color Heliar, as I think this might work out better. If you agree, I bet you'll want the 150mm!
While each of the three will be useful for landscape work, I'd say it's most important not to miss having the 80mm, as it is so very broadly useful. Have fun!
