ASUS has more MHz on CORE CLOCK , more VRAM... and with MANTLE getting really popular on top dollar game engines... i would say : ASUS R9 290X OC. Also, in my country the ASUS R9 290x OC has a lower price than the msi nvidia gpu.
There's no major performance differences. I think it comes down to the individual games that you play. And if they support Mantle, then you should probably lean towards the 290x. But it will not matter across a larger number of games.
I'd advocate going with whatever is cheapest. Which would happen to be the 290 (non-x). Mantle has better results, favourable results, with lower end hardware.
Don't get me wrong, I love Mantle. I want to build my friend a PC with a 7850. But with higher-end GPUs, it just doesn't matter.
The 290x performs slightly better but it will run slightly hotter and louder. If you ok with that then go for it. The 290 will have a lot higher price to performance ratio and perform also identically as the 290x. I'd get that over either option.
That is a DirectCU II. A 290X is phaster than a 780ti with a decent khuler, plus it won't make you crazy and let you fly. A DirectCU II is a very good cooler. People think the ATi cards run hot because those people are too dumb to understand that nobody uses a stock cooler.
WHAT?! Have you seen these things called benchmarks? The 780 ti is shown to be faster in every benchmark and it definitely runs cooler. My Gigabyte Gtx 770 runs at 67c overclocked at load. Compare that to the Gigabyte 280x. It runs at 70c overclocked in online benchmarks. 3c but that scales with bigger graphics cards.