my experience
2x Nvidia 280
2x 570s
2x 7970
2x 7990s
2x 980s
in my experience SLI is much better than Xfire in
Quick Profiles on Day 1 or week 3. xfire sometimes takes weeks-months.
SLI scaling is generally good from 25-100% two examples Overwatch 99% Scaling & witcher 3 is a good 60-80%
Regarding the future Hopefully Game designers go with LDA multi GPU support in Directx 12 and nothing will change NV/AMD will still carry on doing profiles. Some designers like 3Dmark/Ashes that use MDA & have done well, but i trust AMSD/NV to do a better job than Developers.
Personally don't care about DX12. till 1180GTX or The AMD 590. Their wont be any decent games for a while & by then ill have upgraded or close to.
I care about
Witcher 3
Overwatch
Truck Sim
Most can be done on a 980 with ease, But i have 4K so any horse power that can help i want![]()
If you play games that support SLI well then there is no question for you. If the 1070SLI edges the 1080 then i'd get that.