Since you haven't made a decision or maybe you have i'll just throw in my two cents.
Don't sli, it's not worth the hassle, most games aren't designed around sli, and the usage/power scaling isn't linear. So 2 gpus don't mean 2 gpus worth of fps performance its most likely 1.5x at best. Thats if a game even allow sli, or is built around it to begin with. Which in most cases it wont be.
Saying that a max difference between 980 to 980 ti would be 3-8 frames isn't true. At all, theirs tons of factors that go into it. For one, at higher resolutions 1440p-4k you're gonna have a much better experience with a 980 ti on account of the two extra gb's of gddr5. Which will have a pretty big impact on performance for the stock 980. When you buy a non reference 980 ti say the g1 like i have, you're getting quality parts on top of more vram on top of a newer micro arch so you get dx 12.1 instead of just dx 12 with the 980 which in itself could have pretty sizable gains. Also with the ti you're not just paying for the extra vram chips you're also paying for a more solid build, overall my 980 ti g1 oc'ed doesn't peak over 65 degrees under full load.
This is all on top of the fact that the 980 ti will oc, and it will oc much better then a 980. All in all really it's more budget friendly to go with a 980 over the ti. But if you've got the cash and read up on some oc guide's, then yeah go 980 ti if not you know can't go wrong with the 980.
It's not just the extra 10-15 fps you're gonna gain but the overall build quality as well still though thats if you think the 200+ is justified for it.
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If you're gonna go with sli at 1440p with two 970's don't, you wont get ultra settings what you're doing with sli is sharing vram so even though the 970 has 4gb of vram only 3.5gb's are useable, so you don't get 7gb's of vram its' 3.5gb's shared between both cards. From personal experience if you're running ultra settings even at 1080p i've peaked to about 5gb's easy. Even with a moderate oc on both card's you're gonna run into some performance issues with high res textures.