980 or 980 SLI or 980TI GPUs best for P3D?

Hi, I am about to install P3D 2.5, coming from FSX-SE. I have a reasonable system, X-99 [email protected], 980 GPU (EVGA SC), 16 meg DDR4 Memory. Performance on FSX-SE was fairly good, but as I am going over to P3D, which takes advantage of good GPUs, would I see better performance with another 980 in SLI, or swap for a 980ti, (can't afford to buy 2 980ti). I also use the system for other types of sims as well as flight sims and a couple of games. any advice guys, would be appreciated.

Thanks, John.