They are about the same, they are trading blows in benchmarks.
I'd say, however that as a whole, Sapphire has a better deal there, since Dual-X is an amazing cooler (I got 290 Tri-X and the temps are absolutely amazing, so I am sure Dual-X should be as good, since 280 is not as hot to begin with). Also 3GB memory is just a bit more future proof, IMO.
Also, AMD got an edge with their proprietary Mantle API, the games supported list is very small, but there are some heavy hits like Battlefield 4+, upcoming Dragon Age and just about anything that will run on new Frostbite engine. It's a nice nifty advantage, in my opinion.
Also, Crossfire is much easier to setup than SLI if you will want it in the future.
On the other hand:
Nvidia has a superior driver support, AMD is improving, but they are still quite behind on that one.
Another point is that with Nvidia you can use PhysX, which is a very nice eye-candy, but again, it has only handful of games where it is supported.
Also, subjectively, for some reason I associate Nvidia with higher quality. But that is as I said, subjective, especially since out of 3 AMD/ATI GPUs and 3 Nvidia GPUs I owned across these years, I had Nvidia fail twice (8800GTS died after like year and a half and 280GTX deteriorated and eventually died as well) and AMD/ATI 0 times (Radeon 9500 PRO, HD 6990 and now 290 Tri-X). So I guess it is more of a psychological thing, since for me AMD seems to be more reliable.