Best price/performance GPU is still GTX970 even after GTX980 price cut.
GTX980 is only 10-15% faster than GTX970 but about 100-200 bucks more expensive.
GTX980's price can not justify the extra 0.5GB extra VRAM too.
Both GTX980 and GTX970 will struggle on Witcher 3 @1440p if you aim ultra settings with all effects on plus mods like E3FX.
You can also try to OC your GTX970 to reach GTX980 stock performance. (luck is involved here)