So, I had the rb 50, the newest one, and comparing it to the 50 on my mamiya 6, well... no comparison. Edge sharpness was just not there in the rb lens, not any any aperture. Of the wide lenses for the rb, I much prefer the 65. Now, the rz 50 uld has a floating element design that cures the edge sharpness issue. Unfortunately there is no 50 uld for the rb.
If you need the 50 uld, then the easiest thing would be to pick up an rz. You can depin the 50 uld and mount it on the rb, but then some additional surgery on the infinity stops will be required to get infinity focus. So why bother, you may ask? Well, I am a tinkerer and I very much like the 6x8 back on the rb, and I ahve a number of other options such as sheet film backs etc. that I don't have for the rz. If you are new to the rb/rz lines and don't already have some investment in peripherals, and if you really need the 50, then I'd say just get an rz pro 2. If, like me, you have a bunch of rb doodads lying around and strongly prefer an all-mechanical body, then I can give more details on how to adapt the 50 uld.
By the way, I am about to test the 75 short barrel on the t&s gizmo, on the rz and on the rb, so if that interests you, stay tuned. The lens is ~35mm equivalent, in terms of the field of view of a 35mm camera.