Arne,
I only saw this post after first replying in the Mamiya Thread.
I can't really give you a comparison of the angle of the 50mm on the Mamiya 7 to the 55mm on the Plaubel as I only have 50mm on Mamiya 6, just offer my own opinion on the the angle of view I prefer.
For me I think the 50mm would be a little too wide and I would not be too keen on a seperate viewfinder and the lack of the ability to check focus quickly because of the relative lack of depth of field for zone focusing on 6 x 7 as compared to wide angles on 35mm unless well stopped down. I also shoot on the Leica (and lately an R-D1), but my favourite lens on this is the 35mm Cron. I do have a VC 25mm, but the Cron gets the most use for street pictures.
Because of the aspect ratio of 6 x7 compared to 35mm film I do prefer a comparatively slightly wider angle lens for this format for shooting on the street. I do find the 55mm ideal, but if I went to the Mamiya 7 I would check out the 65mm especially as you don't need a seperate finder.
A couple of "documentary" style pictures attached to give you some idea of the angle of coverage from the W67. The Plauabel W67 is a great camera for this type of photography, but the Mamiya 7 should work as well. Martin Parr used the Plaubels for all his early 6 x 7 colour work, but now shoots with the Mamiya 7.
Sorry again about the scan quality of the attached.
Jim