In the long run, it's always better to pick a single strong card than SLI two weak or 'not-so-strong' ones. I'd go for the single 970 without thinking twice, you might be able to sell your current 760 for a decent price today - which is one of the reasons you should consider the single card over the SLI, since if you went 2x760 and eventually were to sell them, their prices would be waaay down the value you initially paid for. This is kinda obvious, and even if you're not looking forward to sell or trade them, it would be best to take the single 970.
And if you have the 2GB 760, beware that VRAM in SLI does NOT combine between cards. If you have 2 2GB cards in SLI, you STILL GOT 2GB of VRAM. The single 970, in the other hand, has 4GB - and consumes less energy as well.