The bottom line is this... you have a 300mm lens for wildlife, on APS-C. If you go full frame, you'll immediately notice the field of view is similar to missing 180mm of extra reach. So you're going to have less on the frame at the same distance at the zoo, or where ever you go to shoot wildlife. If you want to do macro, again, an APS-C will give you more on the frame for the distance. A lot of macro shooters use crops (and even stack TC's in there too) for this reason.
You do not have more len's choice on the 70D, just because EF-S line is included. What EF-S lenses are you wanting that don't exist already in EF form on full frame? This is not something I would worry about. You're not buying all the lenses out there. And you clearly are in the telephoto range, so again, the question of EF-S and EF just doesn't matter as zoom range for EF-S ends early, and the EF range continues further.
If your goal is the sharpest you can get, and the best ISO you can get, then get the 6D. If your goal is to get more animals and bugs filling the frame, without having to buy a longer lens, then stick with the 70D.
I shoot wildlife in the wild, so I go stomping around in Swamps and Prairies here in Florida. I appreciate APS-C with my 600mm lens, because I still feel like it's not enough reach! I still end up cropping down a few photos. A zoo is another story. I could survive with 200mm in a zoo. No problem. But in the wild, I always feel like I need 2000mm. It's just never enough. I do this with macro too. I shoot 180mm prime for macro due to minimum focus distance and 1:1 ratio happening at 18 inches for me. Well, I get even more when using APS-C, as I can focus at the same distance, yet get more perceived magnification. So pretty great macro, at a further distance, means more of the stuff I want without spooking my targets as much (or getting my face in cob webs trying to get within 1 inch of something; I keep a nice 18 inch distance between me and them). For everything else I do, a full frame is better. But for wildlife and macro, APS-C is great. I can't wait for whatever replaces the 7D (I'm passing the 70D this round as my current APS-C works fine for what I do; but I may cave on whatever takes over the 7D in the future, assuming it will have a clean working ISO 1600, as that's all I want, with the focus of the 7D for my wildlife use).