I'd say it depends on your needs. Yes, the 5D3 is way better in low-light, but one year ago there were lots of photographers who used a 7D for wildlife and were happy with it. This things change a tad too quickly, I guess...
This line kind of made me decide for the 60D when I was considering buying it, a while ago:
"Just two years ago, ISO 12,800 was a throwaway setting at best, with the 50D turning out nasty images fraught with overt banding, visible even in a thumbnail. Today I look at ISO 12,800 from the Canon 60D and T2i and have a hard time finding much to comment on. It's impressive. Feel free to shoot in near-darkness."
That aside, I guess it depends on what you want/need the most now. What's the priority, low-light event/indoor shooting or wildlife? Buy the one you need now first, and then save money for the other.
Neuro always points that you don't need APS-C for the extra "reach" if you won't print big, because you can crop images from, say, your 5D3.
Also to be considered is the AF. Is the AF on your 600D good enough for wildlife? (I don't know, but you probably do.)
Tell us what you decided and how you liked it. (I'm considering buying a 7D, too!)