The GTX 980 for sure. The only SLI configuration that beats the GTX 980 is the GTX 970'sin SLI. The GTX 960's simply do not have the power.
Not sure what you want in regards for performance, but I got a GTX 970 now and plan to add a second GTX 970 for SLI later. The single GTX 970 plays any game at 1080p, maxed out, at 60FPS with no issues for me. Then I can get another GTX 970 for SLI when the price drops and increase performance and extend the life of my system without another major upgrade.