If you want at least 60 fps then you should buy the 980 Ti. You will probably get some saying that it's over kill for 1080p but I calll BS on that statement every time I see it. Your current set up is giving you no head room, at 60fps, at all for your gaming needs, and the 980 Ti will give you much more headroom.
I'm in the same boat... do I get a 2nd GTX 980 G1 gaming or move to the 980 Ti....
Decisions, decisions.
@Felix_Bank if you already have a 980 and good airflow, then i would get a second one. My 770s perform a bit worse than a single 980 + all the micro stutter. Then again Vram might be an issue down the road... Maybe the best think to do is nothing till windows 10 hits and then make a move. Who knows, we could see 8gig versions of the 980ti. Thats an instant buy for me.
Good shout @antonymichael
Airflow is awesome and a second one will leave a PCIE gap between the cards anyways, so cooling shouldn't be an issue. VRAM and SLI problems is what i'm worried about.
@Felix_Bank keep in mind since gigabyte cards typically exhaust in the case, that the top card will run significantly hotter than the lower card, in my setup and where i live, in gaming i see the top card in the 75-80C temperature range while the lower sits at 60-65C (3ple fan cooler). Another thing to look for is to get the same revision and model of the card, preferably from the same company. Even though sli can work if the cards are from a different manufacturer, it would be a shame to lose some clock speed (unless you OC). It is also advised to have the same bios revision between the cards.
Generally i wouldn't recommend sli, its an enthusiast thing, you have to be prepared to tinker with settings, frequently clean the case to keep the temps down, reapplying the sli option each time you update the driver. Not that its hard work or anything, considering the gains. Most of the time, you gain at least 1.7x the performance versus a single card. Microstutter is a non issue with the 980 for current games at least.
What hurts the most is the decision on when to upgrade. Do you wait for a card that does X fps more than both, double the fps, or vram limitations. Depends on budget i guess.