thank you ... changing to a nikon system doesn´t make a sense too me as i have 9 prime lenses from canon. by the way the nikon 810 is a great instrument.
for me, medium format has always two special things over the megapixels - the depth of field and dynamic range (shorter sync time too)
but as i read more and more about the canon 5dr(s) it should be the right instrument for my pixel-development in the studio.
(and having a good light also improves sharpness and quality.... love my broncolor-lamps ;-))
Your Canon system is already top grade and I would say the 5DS(R) is a milestone in the 135 format. It has forced the 2nd-hand prices of medium format digital backs to drop, just like the D800(E) did 3 years earlier. If Canon suits your shooting style then I see no point to mess with other systems around.
Advantages of the Canon system:
a) Access to incredible wide angle lenses, such like the 17mm TS-E, 24mm TS-E, 11-24mm L, 16-35mm IS, 35mm L II etc. You will find no such lenses with matching performance in other systems for now.
b) Far better autofocus performance than medium format.
c) Far better low-light performance than medium format thanks to faster apertures. The aperture of the medium format lenses are generally slower and when you take this into consideration, Canon wins in image quality thanks to lower ISO settings.
d) Better DoF control than medium format thanks to faster apertures. Below shows for the same model with height 170cm tall, if you shoot her in portrait orientation from head to waist, how large the circle of confusion can get (normalized for the frame) with respect to distance behind the model. Here the crop factor 0.79 is based on the 44mm x 33mm of the CFV-50C, and the crop factor 0.64 is based on the 54mm x 40mm of the Phase One IQ3 100MP. As you can see, normal Hasselblad lenses will not match the degree of bokehliciousness of the Canon lenses. Even if one manages to mount the legendary Hasselblad 300mm f2.8 onto a Phase One IQ3 100MP (probably via ALPA 12 FPS or something else) it is still defeated by the Sigma 200-500mm f2.8 lens mounted on a Canon 5DSR. The only way for medium format to win this bokeh battle is to use the Zeiss 1700mm f4, which is not affordable to most photographers on the planet.
Disadvantages of the Canon system:
a) The f1.2 lenses will get the bokeh cut thanks to the blockage cause by the mirror box of the digital Canon DSLRs. This will effectively reduce the degree of bokehliciousness. If you mount the f1.2 lenses via ALPA 12 FPS onto a Phase One digital back e.g. IQ260 you would see that there will be no cut of bokeh. I don't think Canon will address this issue.
b) Dynamic range: if this is really important to you then Canon is not a good choice (and CCD digital back is not a good choice either). Currently the best rated Canon camera in terms of dynamic range is indeed the G7X, which features a 1" Sony sensor inside. Pixel-wise, i.e. without normalization down to 8MP, the dynamic range of the 5DS(R) is still the same as the old Canon technology e.g. 7D.
Even Dpreview has criticized the dynamic range of the 5DS(R): there can be scenes where DR is not within your control even if you have the most expensive and largest Broncolor with you.
Personally I have compared the 5DSR against the IQ250, which uses the same Sony IMX161 sensor as in the CFV-50C and you can see the dynamic range difference here in terms of shadow noise:
You could further check the dynamic range ratings by Bill Claff here:
Photographic Dynamic Range versus ISO Setting
I would say that if dynamic range is important for you then get a camera with a Sony sensor inside, e.g. Phase One IQ3 100MP, Hasselblad CFV-50C, Nikon D810, Sony A7R-II etc.
You could also wait until Pentax and Hasselblad gains access to the 100MP Sony CMOS sensor in a couple of months (presumably, according to rumors).
I know quite a few people who sold out their Canon glasses and switched to Nikon back in 2012 when they first saw the shadow recoverability of the D800E. If you are not willing to switch (and I don't think you have to) then the Sony A7R-II provides a great alternative option. However due to its BSI and 5-axis IS design, the dynamic range is not so good as the Nikon counterparts, which is especially true for long exposure. You could read more in Jim Kasson's test results here about the overheating:
a7RII self-heating for long exposures: Sony Alpha Full Frame E-mount Talk Forum: Digital Photography Review