considering the lack of Physx supporting titles out there, SLI would be more beneficial to you.

Karamses, you need to do some tests. This "subject" will always be debated. There are far too many benchmarks out there that show that you "DO" get a boost in FPS, while others will say that it's not worth the time or effort to have the card in the computer, and just go with SLI. This is true, in most cases, but when you do have that physx card in your pc, again, you will get an FPS boost, but only in the titles that support it. It's your call, but never buy a physx card.
 
Another question you should ask is "would it bottleneck my pc?" Again, another argument. go in and run some heavy physxs games (Assassin's creed black flag, all batman games, lords of the fallen (another yes). Max out the physx. then go back without the physx card, and see what kind of results you'll get.
 
 
Edit : just for the record, when i only had a single 970, I was using my old gtx 470, and I did get a jump in FPS, and things were smoother.

you're gonna want the horsepower with 2-Way SLI for 1440p.

Grab a second 970 FTW and be done. (Don't get a FTW+ as FTW & FTW+ cannot SLI).

1440P is gonna start pushing the boundaries of the 970 FTW with its 4GB memory. Pushing into and beyond 2K will expose the insufficiency of the 4GB memory and breaking into 4K blows the issue open. 970 is a great 1440P 2k card. get the FTW model so your only about 1-8FPS off from a 980 performance on average and rock it. If you plan on going Beyond 2K res, upgrade to something with at least 6-8GB later.

Thanks, went for the ftw, will post benchmarks when it arrives

I run a 4K setup running a singel GTX 970 SC. You just need to turn down your settings somewhat. But you can find playable settings that are still mainly on ultra or high.