I switched from the 5D3 last summer to the 5DS and have never once looked back. Yes, I can tell a huge difference (others may not have) in several areas.
1. Resolution of course, you can crop an image for days with 50MP. But on top of that the image, if exposed correctly, are amazinly sharp. The amount of detail it can resolve is great. I read someone on a forum mention you should shoot it like a crop camera meaning to keep you shutter speed up to avoid as much camera movement as possible and I find this faily true.
2. White balance is much better than my old 5D3 for some reason. I've RARELY had to adjust WB since getting the 5DS. Add on top of this the anti-flicker tech it has and you won't have to mess with WB again.
3. Auto focus is improved, the 5D3 was no slouch but they did improve it.
As far as upgrading your lenses you look to be in good shape, your 24-70 might benefit from going to the version 2. It won't make a good/sharp lens worse but it will show off any weaknesses it has.
Another benefit I've loved about this camera is the 1.3 & 1.6 crop mode. If you can stand taking a hit in framerate(only 1 FPS less than the 5D3) I promise you that you won't touch your 1D3 ever again. I shoot middle school soccer with mine and the 70-200 and it really shines.
The only downside I've come acros is the file size while working on my computer before I upgraded it. With an average RAW size of 70MB it can tax a good system pretty quick.
Get one and don't look back...