I brought a list of questions to ask about the 5DS and 5dsr... and oddly enough, it's not something I want or need. Nice piece of hardware, but for what I do, not of any use.
Larger megapixel sensor means it’s more sensitive to camera vibration. Since most of what I do isn’t studio, or landscapes or other tripod mounted shots (photographing birds, wildlife, street) tracking erratic flight of birds, if using 5DS/5DSr, it would cause a lot of sensor vibration, and screwed up shots.
Also, I asked "If I want a 50mp shot, I will take 4 photos, import them into layers, align them, blend them.", and he honestly added that I would get the same results, but with studio work, you get that all in one photo. Fair enough, but I don't often do studio work or landscapes.
After asking all the questions I had on my list it comes down to this: The new Canon 5DS and 5DSr are great pieces of hardware. But what I've got now does everything I want and more.
Finally got my hands on the new Canon 11-24. Focuses surprising fast. Only got to use it indoors, though. Will have to hire it to try outdoors.
The 300mm Prime is ok, but I have 100-400. True, it's 'only' an F4, but I am normally shooting at f/8 or above.