Graphics cards in SLI can create a lot more heat, noise, and pull a lot more power. Especially ones from previous generations. The 780 ti's in SLI will definitely outperform the 980 GTX, but can definitely have microstutter issues that can make gameplay not as smooth. While most games can utilize SLI these days, I will always go for the powerful single GPU setup now becasue then I can SLI it in the future. I would only SLI a 780 ti if I already owned one 780 ti in the first place. The 900 series cards also have new technology as they have Dynamic Super Resolution that renders a game at a higher, more detailed resolution and intelligently shrinks the result back down to the resolution of your monitor, giving you 4K-quality graphics on any screen. The 700 series GTX cards can't do that.