Generally, I'd prefer Nvidia, they just seem more efficient and cooler.
However, I got a Sapphire Tri-x R9-290 because;
It was cheaper in Oz
I run 3 monitors and it has 4GB Vram, I don't believe 3GB is enough with BF4 already using 2.7GB and Titanfall using all 4GB (despite the crap graphics), I'm sure others will follow suit.
I wanted the Sapphire for it's flex DVI outputs, I had issues with DP to DVI conversion, flex does not need this. I didn't know if Nvidia did or not.
The Sapphire cooler is amazing and very quiet. When gaming, I can barely hear it, even if the GPU is pushed to 100% and it holds the temp around 65-70C. Furmark pushes it up to 82C @ 51% fan speed, which you can hear, but isn't bad.
For the modern games I play (mainly FPS), all the benchmarks seemed to favour the R9 over the 780 with the odd exception, in fact many favoured it over the Titan.
AMD are generally more allowing of over voltage/ over clocking than Nvidia, although I did find a good BIOS editor for my GTX680.
I may want to crossfire in the future and AMD's PCIe solution may be better than using a bridge, although I've not looked into it so much.