Price performance, R9 280
Don't care about price and stay at 1080p, 960.
Want to play at above 1440p or use custom ultra high textures or down sampling from very high resolution, 280.
I recommend 285 though, since it's not a rebrand like 280 and it's based on Tonga( DX 12/GCN 1.2, freesync, etc). TDP also not as harsh at 190 W.
I would not reccommend 285. Higher pricing, similar performance to 280, but less powerful raw specs compared to 280 (narrower memory bus, smaller amount of vram, stream processors too I believe) more energy efficient.
Buy 2 cheapest 280 n crossfire
Like that might as well buy a single r9 290x lightning or an atas gtx970.
A dual 280 will just yield a single 290x performance.