GTX 770 4GB SLI or GTX 970 for 1440p?

The twin 770s will be faster but require more power, they also have less life in them. with pascal about to drop, it's questionable as whether it is wise to invest in 2 generations old technology.

According to techpowerup's 19 game test Suite, SLI scaling averages out at about 70% ... that includes the tare games that don't have a SLI profile.....Scaling in games that need it, Tomb Raider for example, scales at 97% at 1440p. When a game doesn't scale well, you generally get fine performance (~60 - 100 fps) with just a single card. Where SLI comes in handy is when ya getting < 30 in Romb raider and it takes you to up to or above 60 fps depending on OC.

Tomb Raider goes from 29.8 to 58.7; scaling = 96.98%
Battlefield 3 goes from 62.1 to 121.4; scaling = 95.49%
Far Cry 3 goes from 35.6 to 68.8; scaling = 93.26%
Crysis 3 goes from 22.5 to 43.3; scaling = 92.44%
Thief goes from 70.8 to 136.1; scaling = 92.23%
Bioshock Infinite goes from 76.7 to 143.9; scaling = 87.61%
Splinter Cell: Blacklist goes from 49.5 to 92.2; scaling = 86.26%
Battlefield 4 goes from 45.0 to 83.2; scaling =84.89%
Metro LL goes from 40.7 to 74.6; scaling =83.29%
Batman: Arkham Origins goes from 81.8 to 148.3; scaling = 81.30%

Again, according to Techpowerups 19 game test suite, the 970 is faster than the 390/390x at 1080p. When both cards are overclocked, the 970 is also faster at 1440p than both the 390 and 390x, tho the edge over the 390x is just a hair. This is because the 390x for example has just 6% overclocking headroom while the 970 has just about triple that at 17%.

There is no game that is made today that in any way, shape or form benefits in the slightest extent from more than 4 GB of RAM. While you can start to realize a benefit at 4k resolution, the problem is, that any game that can break 4Gb can not be played at the settings required to do so at 30 fps. In short, the card might benefit from more RAm, if the card could actually run the game at playable frame rates ... no such card exists.

http://www.extremetech.com/gaming/213069-is-4gb-of-vram-enough-amds-fury-x-faces-off-with-nvidias-gtx-980-ti-titan-x

While we do see some evidence of a 4GB barrier on AMD cards that the NV hardware does not experience, provoking this problem in current-generation titles required us to use settings that rendered the games unplayable any current GPU. .....

The most we can say of a specific 4GB issue at 4K is that gamers who want to play at 4K will have to do some fine-tuning to keep frame rates and resolutions balanced, but that’s not unique to any vendor. If you’re a gamer who wants 4K and ultra-high quality visual settings, none of the current GPUs on the market are going to suit you.

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In short, if money was no object I'd get a 980 Ti ... if budget restricted, Id get the 970 and use that until you can afford a second one.... one 970 is a bit weak for modern 1440p monitors as ypoiu can see by the tabulated results above.

Finally, I would consider rethinking the position on 60 Hz.... the new IPS 144hz monitors from Acer / Asus are simply incredible ... especially if you can use ULMB.