SLI isn't the best software, I'd always take a superior card over 2 worse cards, even if they vow to get you more "performance".
But in reality, those are laptop grade GPUs and aren't the best anyway.
i have had both configs and i say go for the 780m. When overclocked they are both about equal however the 780m is a more simple solution. I used both in my alienware 18 when i had one.
SLI isn't the best software, I'd always take a superior card over 2 worse cards, even if they vow to get you more "performance".
But in reality, those are laptop grade GPUs and aren't the best anyway.