Good luck! I have a Mamiya 7-II, and use the 43 mm lens occasionally. It is a great lens, but use it with care since slight deviations fom the horizontal cause rather substantial distortion (bowing of the horizontal) particularly at the edges of the frame. I looked at the Fuji 690 III, but decided against it due to the fixed lens design, and lack of buit in light meter. The Mamiya is a great camera! I use it for street photograhy since it is quiet, and not all that large (considering the 6 x 7 film size. The light meter is very accurate. There have been some reports about difficulty in focussing the 150 mm lens, but I have not seen that problem.
I have found the 65 mm, and 80 mm lenses to be the most frequently used. Mamiya even makes a polarizing filter that resembles the Leica design.
Once you use it, you will love those huge negatives (or slides), and will have a hard time going back to 35 mm.
Best of luck in your choice.
Martin