Many prefer a single fast card over two slower SLI cards. Two 770's in SLI will give you better performance for a cheaper price for sure.
I was in the same situation, owning a single 680 and opted to simply purchase another 680 to use in SLI. The performance is fantastic and I feel like I got the best value.
However, I do have to change the settings in the Nvidia control panel whenever i surround game, to a mode that uses both cards fully. When i'm in desktop mode and want my start bar to span all 3 monitors, i have to switch back to a surround mode for the desktop. For some reason, the software has to confine the taskbar to one monitor while in surround gaming mode vs just desktop surround mode.
You will also notice that many games, upon release, may have driver issues with SLI at first. Sometimes this affects performance where the game may actually run slower in SLI mode than using one card. Other times, you may find graphical bugs and such. Eventually drivers get sorted out and usually the games run well.
This is basically just life with SLI. You get some monster performance, but if you're annoyed by little bugs and things, prepare for them.