DayZ is a crazy mess. I play it alot, but its rough. No matter which card oyu get, don;t expect to max it. It is simply coded far too poorly has and too many vRAM and memory leaks. I have an i5 and a gtx660ti and I struggle to keep 30+fps at normal settings.
Either card will be great for DayZ. Just know that it is not hardware limited at this point. My CPU and GPU never go over 60% usage but I can't lock the game at 60fps. It is an Alpha game. Thats what we get.
If I had to pick, I would probably go for the 280x. It is slightly cheaper and I have been with nVidia for a while now and I would love to try the 280x out.