I decided to go for the 5Dsr, here's my reasoning at closer to the time:
https://www.dpreview.com/forums/post/57449241
I find I get aliasing or moire very rarely, however I spent a while looking at how several tools handled it. Note that I think DXO have improved since then:
https://www.dpreview.com/forums/post/58034634
BTW if you look at the 1:1 crops here:
https://www.dpreview.com/forums/post/57507061
You can spot a little moire buried in the hair, although really only forum nerds would worry about it in real life.
On the subject of detail I would recommend using the Fine Detail Picture Style for JPEGs as the default, ignoring Standard. It just has a better sharpening algorithm.
I have the W-E1 card and keep it in the SD slot. I find it works well, although you always seem to have to do two clicks more than you should when enabling it (don't miss the last press). Also my iPhone 7 handles the 50MP JPEGs okay and the image processing programs I use on it also work with them (e.g. SnapSeed). I use both the Canon App and Cascable, usually the former for image transfer and the latter for shooting. I haven't done the "using a Tablet for customer preview" thing though, not sure how well that would keep up. On that subject...
I shoot Raw and a large medium JPEG (large fine JPEGs can get stupidly large). Reckon on an average of maybe 76MB per image (9MB for the JPEG, 67MB for the CR2, less if birds against blue sky or similar low-content images). The largest Raw file I have is 94.2MB, you won't see many below 50MB or above 80MB. The largest "large medium" JPEG was 20MB, but that's quite unusual.
Note Memory Card write speeds top out at about 100MB/s so you can fill up the (14 shot) buffer even at just a couple of shots per second if you go on long enough. In-camera write speeds are available here:
http://www.cameramemoryspeed.com/canon-5ds/sd-cf-card-comparison/
I have:
SD: SanDisk Extreme Pro 95MB/s 128GB
CF: Lexar Professional 1066x 64GB
Which I have had no issues with.
The most I've shot in a day is just over 50GB (birding), but with some video I could push that up a bit, hence my decision to drop from 128GB to 64GB for the CF card.
I don't have issues with hand-held shots, the pixels are larger than the 24MP APS cameras, so if people can hold them steady...
The video is only FHD (1080p) but if you use the Neutral profile (or the Technicolor CineStyle profile in a Custom Slot) it's very clean, also it does well at night (the video, that is). I don't know what they are doing but it's cleaner than both my GH3 and GH4 (shooting 1080p, the GH4 is on a different planet in 4k). The rolling shutter is 27.7 ms (5Dmk2 is 25.9).
Other thoughts:
Live-View shooting is a bit of a pain with long blackout times. It does shoot the image quite promptly though.
Video users might be unhappy there isn't a clean HDMI.
If you use the crop modes it only crops the JPEG, there is no option to crop the Raw so you don't gain much buffer-wise.
What I'd like in a mk II (but probably won't get most of):
DPAF
In-camera image tagging
Deeper pixels (so lower ISOs possible)
The new AF point illumination scheme
Improved AFC subject tracking
Built-in MFA
Cleverer VF overlay (a hybrid OVF/EVF would be perfect)
In camera shooting for focus stacking
Faster memory cards and faster write speeds
Much deeper buffer
Much better Live View shooting experience
More DR (the mk I doesn't suck though)
Spot meter possible at AF point used
Electronic shutter mode in Live-View, so mirror stays locked up
More options for what you can program onto configurable buttons
Clean HDMI
Built-in and fast WiFi (802.11ac AC1800 or faster)
Bluetooth
4k video so I don't have to carry a second camera if I want that
Built-in radio flash controller
Maybe a fully-articulated touch-screen rear-LCD
Possibly a few more pixels, not really a big thing though
Hope that helps. I've been delighted with mine.