Are crop sensors still relevant or should I just go FX in cropped mode

I'm doing a Safari trip to Africa later this year, and I would like a backup body for my D700, to which I will attach a telephoto lens. Initially I was going to go DX for the crop factor, but I'm finding a lot of comments online that DX is becoming less and less attractive in light of the new FX cameras which pack a lot of pixels and are very good at cropping. The D600 comes to mind. Sure it's basically twice as expensive. But money factor aside, is there still an advantage to the D7000 vs the D600 in cropped mode when it comes to quality?