First thing, single Nvidia GeForce GTX 980 is faster than the AMD Radeon R9 295X2. Benchmark also shown that it have very little difference of "losses" with 295X2. Next, AMD Radeon 295X2, is a Dual GPU Graphic card. So the RAM is shared equally which means 4GB per GPU. Just imagine being SLI-ed. Nvidia GeForce GTX 980 being SLI is a clear winned. To add more, a single GTX 980 beats the GTX 780, GTX 780 Ti and AMD Radeon R9 290X. It's the fastest single GPU Gprahics card now. With that being said, rumors are AMD will launch AMD Radeon R9 390X to compete it and been said that it will beat it. Will be release in February 2015. An exciting moment and fear moment too. To be honest, Im a Nvidia fan. While that is true, Im not a complete fan boy. I like Nvidia because they are fatser and have so much technology support. Like 3D suroorund, VSync, Shadowplay, Cuda etc. I also learning stuff that need these support. But choposing the correct winner is uneasy. There no winner. Just faster or better. Also, overkilling is waste of momey except if you plan to use it later. An AMD Graphics card or more precisely, AMD whole, focuses more to gaming. Intel and Nvidia are more to server, 3D etc and of course, in some games, it's an overkill. So, if you were to play games, on a 4k or 3 display, both Radeon R9 295X2 and Nvidia GeForce GTX 980 are good with pricing with more to Nvidia since they also have those supports. But if to say Nvidia increases the price, and you are playing games on 4k or 3 display with no plan to use the supports, I would advice to go AMD. But if you want to go 3D etc, Nvidia is the way to go.
Lasty, it all comes to what you want to use it for. Good luck