Where are you finding 5DS-R for $1500?

I see them selling new for $3700 and used for around $2400, at a minimum.

I'd be seriously tempted if I could find one in reasonable used condition and with moderate "mileage" for $1500!

But the 5DS-R is a pretty specialized camera... as noted:

- Limited ISO range: 100-6400 versus the 5DIV's 100-32000 (I never use "extended" ISOs).
- The 5DS-R uses dual Digic 6 processors... The 5DIV uses a single Digic 6+ processor and an iTR AF processor.
- DXO rates the 5DS-R with less dynamic range and less high usable ISO, compared to 5DIV.
- 5 frames per second maximum versus 7 fps with 5DIV.
- -2EV capable autofocus versus the 5DIV's -3EV.
- Less sophisticated AF systems, including:
--- Slower contrast detection AF in Live View, versus the 5DIV's DPAF.
--- Lower performance viewfinder-based AF array vs 5DIV, which has added processing power dedicated to AF.
--- Fewer images can be buffered (due to larger size?) compared to 5DIV.
- Both cameras have 61-point AF systems, with 41 points the higher performance "cross type". However, the 5DIV's array covers slightly more of the image area.
- 1 million pixel rear LCD versus 1.6 million pixel Touch Screen on 5DIV.
- HD video versus the 5DIV's 4K.
- 700 shots per charge, versus 900 shots with 5DIV.
- No built-in WiFi or GPS, both of which are built into 5DIV.
- Very large 50MP images may slow down post-processing, compared to the 30MP from 5DIV.
- Larger image files will also fill memory cards and hard drives a lot faster.

Actually the two models share a lot more than they differ.

Neither camera has an articulated LCD screen (which the 6DII has), but an external screen can be tethered to either of them to provide a larger view from any angle.

As noted in one response, the 5DS models' high resolution is quite demanding of lenses and unforgiving of even slight camera shake (mirror slap). Back when Canon first introduced the 18MP APS-C cameras they released a white paper about how the greater density sensors were calling for faster shutter speeds to counteract susceptibility to even slight camera movements. Resolution has only increased and pixel density if even higher with the 5DS models. (Sensor pixel density is lower with the 30MP 5DIV.) Tt's going to be interesting to see how the 32.5MP APS-C 90D and M6 Mk II, as well as the next generation of ultra high resolution full frame cameras perform in this regard!

In conclusion, the 5DIV is a more versatile type of camera, more of a "generalist" and less specialized than the 5DS-R. I'd expect the 5DIV to be a better choice for sports or wildlife photography, while the 5DS models may be a better choice for landscapes and portraiture. The 50MP camera is more likely to require a longer exposure and a tripod, especially when used with available light, as well as greater care to avoid shake such as mirror lockup, a remote release or self-timer delay, and/or Live View/tethered shooting.

Rumors are that the 5DS models will be replaced in 2020 with one or more R-series mirrorless... And, further, that we might expect around 83MP (in full frame this is the same pixel density as the 32.5MP APS-C sensor). Who knows what we might see in a 5D Mark V... If there even is one. Maybe it will be an EOS R Mk II, instead. I would be surprised, though, if a 5DIV replacement were any more than 35MP to 45MP. More than the 32.5MP APS-C, but nowhere near as extreme as the 5DS replacement is likely to be. (Note that the rumors about the 1DX Mk III coming next year suggest it will either be the same 20MP as the Mk II or a modest increase to no more than 24MP.... Canon isn't necessarily going for record-setting resolutions, in spite of the big boost seen in the 90D.) This is pure speculation about a 5DIV replacement though... there haven't even been any rumors about it yet.