My pennorth here would be that I have a 7DMk1 and it is woeful in bad light. Even in decent light, I have to expose right to bring some control over the noise. If you can live with noise then that's cool but I prefer clean images. Banding with the 7d1 can also be an issue.
That said, I have no experience of the Mk2 and it is by all accounts, a much better sensor than I have in the Mk1. Even so, a quite well known wildlife friend of mine was telling me a few weeks ago that the Mk2 is "no low light camera". The Mk2 is certainly an improvement over the Mk1 but (here comes my point) it won't live with an FF camera in low light. The FF should also have better general image quality but that will only be noticeable at pixel peeping levels.
I have a similar dilemma to yours, do I upgrade my Mk1 to a Mk2 or do I go FF? The 5dMkIV is out of reach for me at the moment so it would have to be a 6D or a 5dMkIII. Of the 2, the MkIII is the better camera vs the 6d regarding how it does things but the image quality is no better than from a 6d.
With my dilemma, I've come to the conclusion that it's horses for courses. The 6d would make an awesome landscape camera and in the right hands, is quite adept at wildlife. The 6d won't live with the 7d (1or2) though for wildlife in the right light. The 5diii would do both landscapes and wildlife, as well as other genres very well. Being as I already have a 7D1, it would be an expensive upgrade to a mk2, so my next body will be FF and then I'll have both, albeit with a noisy 7d1.