They would be on par with one another but neither will be able to handle high settings on 3 monitors. So its better to stick with the 660 Ti's if your not doing an actual upgrade
Hi
It very important what you gonna compare in both cases - Game , 2D ....
Also if it`s GAME what settings level you will compare .......
I expect both perform close to each other but on 780 you can have 2-d one for SLI