Lots of ifs on tihis one, but here's my thoughts.
I've been looking at both of these for years now and have decided that if I do get one it would be the M6. The benefits to the AR7 are that it floats (very beneficial in your application) and that you can take a follow up shot if necessary. But I've always thought that if things were so bad that I needed the weapon for survival, it would be just my luck for the thing to break or jam. Bad thing for the M6 is that it's single shot, nice thing is that there's nothing to break or jam. I also like the versatility of having the .410 and the .22 in the same package. No, it's not as good as a dedicated rifle or a dedicated shotgun, but it appears to be good for what it is. As far as the caliber, I've shot both quite a bit (in diffferent firearms), and I don't think the Hornet gives you enough advantage over the LR for the weight and especially the price difference.
But it really comes down to why you take a firearm at all. If it's for survival hunting, then the M6 or AR7 might be a good choice, but if it's for defense against two legged threats, I think you're better off sticking with your 870. I don't even know how many types of 870 there are (quite a few), but if yours is a traditional hunting gun, you might want to think about putting a shorter barrel and synthetic forend and folding stock on it. I've done this to mine and it makes a big difference in weight and storability. I don't know about all the regulations up in NY, but it's probably a good idea to check on the stocks if that's the road you decide to take.

Hope this helps,

Chris.

Folding stock and forend

Interesting M6 setup