760 SLI > 770
But if you are playing in 1080p then you won't notice a difference between the two, so if you have a 1080p monitor, then you should go for the 770 as it is cheaper and it can be SLI'ed later for epic performance which would definitely thrash the 760 SLI of course.
So we can conclude that :
1. 760 SLI > 770 when it comes to power
2. 760 SLI and 770 anyone would be awesome for gaming at 1080p. Almost no difference.
3. 770 can be SLI'ed later for much better performance when needed (maybe after a year).
4. Single 770 is cheaper currently about 80-100 dollars (depends on which manufacturer's card you plan to SLI)
So, my recommendation for now would be a 770 because for 1080p there is no difference at all between both of them. But if you want to play in resolutions higher than 1080p, then go for the 760 SLI.
I hope this answers your question. Is there anything else I can help you in?