Two 960's have more power than a 970, with the same amount of RAM. Obviously the game you're playing changes the SLI support, or the monitor you use changes the need for two cards over one. If you have a 120hz 1080p, I'd say go SLI 960. But above that, or with a 21:9 ratio, I'd switch over to the 970.
I'd add, depends on what price you're getting the second card for. Does saving a $100 equal questionable SLI support? or do you play one game all the time that you know has great support, and you need high FPS?