The low light capability has come in handy multiple times. There have been times I have taken the 6D and have run into it being too slow for the moment.
Sounds like the 5DS wouldn't be a good fit then. The 6D has excellent high-ISO performance. The 5DS offers vastly increased resolution, but the tradeoff is one stop worse high ISO ability (similar to the 7D2). You would take a step back in this regard. If you shoot a lot at high ISO, this is NOT the camera for you. (Also factor in the higher shutter speeds needed with the 5DS to get sharp shots at full res - demands more light/higher ISO too.)
6D/5D3 @ 6400 = 5DS @ 2500/3200. (Note: having said that, in some shots enough detail is captured that you can apply enough noise reduction to get it down to 6D level and maintain similar levels of detail....but then again, why bother with the 5DS then?)
For general shooting, I would go for a 5D3 (and that's what I use myself). Maybe wait to see what the 5D4 offers if you must have the latest.
Also, after playing around with a 5DS at the store, it seems you really do need the latest/sharpest lenses to take full advantage of all 50MP. Mostly primes - the 100 Macro and the 50 Art (Sigma) were the only two lenses that I felt really did the camera justice. As far as zooms, the 70-300L was just OK and the 24-105 and 70-200 were awful. And all three of those lenses are decent performers on the 6D!
Not to be rude, but do you have the 5Ds, or did you just play around with it at the store? Because everything you said about it was pretty much wrong. And I hate to see misinformation given to someone asking for advice.
First, the noise is better than the 7D2 at high ISO. You're forgetting that yes the pixels are the same size, the 5Ds has a larger sensor. I agree the CFA is stronger though because I notice the colors are better so that may reduce some of the QE.
Your comments regarding the lenses are not correct either. We've had tons of discussions on this topic and it still appears people aren't understanding. You will always gain resolution. Crappy lens on 5Ds still produces more detail than same lens on 5D3. Period. No exceptions. And what 70-200 lens are you talking about? Hopefully not the v2 IS lens.
You didn't mention that the AF system is every bit as good as the 5D3's too.
Is high ISO as good as 5D3 or 6D? No, of course not. But at least the OP has all the facts now.
Never said I had
the 5DS, thought I was clear when I said I was just testing it out.
As for comparison of noise levels, I meant at pixel level. But here's where you (the OP) can investigate the difference with your own eyes and be the judge:http://www.dpreview.com/previews/canon-eos-5ds-sr/5
You can select camera models and compare.
Honestly, it does appear to me that the 5DS has slightly more noise than the 7D2 BUT the pattern is finer (smoother), colors are better and detail is greater, making the 5DS result more pleasing to the eye.
And in terms of resolution, yes, you can pull out more resolution on the 5DS than the 6D/5D3 regardless of lens. The worse the lens, the smaller the resolution difference will be (Lens Rentals has a good write-up here http://www.lensrentals.com/blog/2015/06/canon-5ds-and-5ds-r-initial-resolution-tests
). But the point I'm getting at is this: if you are going to take full
advantage of the camera's resolution - if
you blow your images up to 100%, crop heavily and/or print big - then you are going to want high quality lenses...there's really no getting around that TBH. LR showed the increase in resolution with the 50 1.4 (a mediocre copy, apparently) to be 20% on the 5DS over the 5D3...but your image size goes up accordingly by almost 51%...meaning that IF you view your image at 100%, it will appear softer on the 5DS. But if you downsize the 5DS pic to 5D3 resolution, it should be sharper than the native 5D3 shot. Depends on what you want I guess. Also, image processing times will be longer as well (if you shoot RAW).
The 70-200 was the 2.8 IS, original version. (They didn't have version 2 unfortunately, I'm sure that would have been significantly better)