3.5GB would work fine for 1440p and it also depends on the game. BF4 uses lots of VRAM while Witcher 3 uses a lot lesser and produces great graphics. It's all about developers optimizing and doing a great job on their codes.
Instead of 970 SLI, a single 980 Ti would do the job and it's always better to have a single card setup than SLI. You only go SLI if;

1. A single card solution is unable to give you enough juice
2. Cost is a concern; 2 x 970 SLI is cheaper than a 980 Ti
3. You just want the looks of SLI

SLI might (not always necessary) introduces various issues like scaling, stuttering, etc. Unless you plan to buy now and stick with your 970 for 2-3 years, there's no point worrying about "future proof" because eventually you end up upgrading and future cards would come with more VRAM. In technology, the wait never ends because if you keep on waiting, you will never buy anything The only reason why 970 becomes cheap is because there's something new and the cycle continues.

If you hadn't have a 970 or the mobo don't have good SLI support, I will always suggest single 980TI.

The best 970 SLI can scale is just slightly better than 980TI, while having 2.5GB less effective VRAM.

Price wise, they are pretty close together too.