Ultimately, it comes down to what kind of power you need. I recommend searching for benchmark results on both setups, and look at the games or other applications you are interested in running on your new graphics card(s). I suspect that a single 770 is enough.
A single card is more energy-efficient and a simpler thing to configure. While it is true that SLI setups are especially well-suited for multi-monitor (3+ monitors), any SLI configuration is limited to only two monitors when in maximum performance mode.
I have run both powerful graphics units alone (like the 8800GTX) and in SLI (2x470's) in the past, and still do, these allow me to use all of my four monitors.
If you have two monitors or less, I would suggest you to have a single powerful GPU rather than pair of weaker ones. That's because using SLI is just an unnessesary risk, as it could be far less consistent than a single one.