1. SLI is a non-issues until 10xx series when nVidia put the nerf on because the only thing it was competing with is 1080 / 1080 Ti
2. VRAM a non issue assuming is 1080p. If you want to look at the data, compare the 3 GB and 6GB 1060's. The 6 GB will always be faster cause it has 10% or so more shaders. At 1080p, the 6 GB is about 7% faster overall. If VRAM was an issue, then we could expect to see that the performance difference at 1440p and 2160p would be much greater as the 6GB would begin to show its advantage. But it doesn't have an advantage. Compare the 6GB and 3 GB performance data for the MSI 3GB and 6GB versions and you see that the performance advantage at 1400p and 2160p hardy varies from 1080p... usually 1% and sometimes 2% as resolution rises from 1080p to 2160p.
The misconception arises cause folks don't understand the tools they are using. GPU_z, Afterburner and everything else is simply not capable of measuring VRAM usage. It measures VRAM allocation. Install a game on a 1060 3Gb and it may allocate 2 GB; install it on a 6Gb 1060 and it may allocate 4 GB... simply because "it's there". It's kinda like having a credit card with a $5,000 limit and $500 in charges on it ... when you apply for a loan, $5,000 gets reported to the credit agency, not $500... GPU-Z and all the others are simply doing the same thing.
https://www.extremetech.com/gaming/213069-is-4gb-of-vram-enough-amds-fury-x-faces-off-with-nvidias-gtx-980-ti-titan-x
3. That being said, the 1080 scores a 96% in the chart below, the 780 Ti scores a 60%. I don't recall 700 series scaling but with 9xx, average was around 70%. The 1080 is about 60% faster so scaling would have to be 60% for the 780 Ti SLI to make sense. I don't think that it is.... I have a faint recollection that it was in the mid 50s.
4. Unless AMD brings some competition to the market, nVidia would be out of their minds to improve SLI performance at lower resolutions. Two 970s was the same price and a 980 and it was, on average, 40% faster. So SLI was a no brainer. Now, with the 1080p - 1440-p nerf, we only recommend SLI at 4K where scaling is > 50%. At 1080p, it's just 18% and at 1440p it's just in the low 30s.
5. I am assuming you have a 780 Ti already .... right now a decent 1080 is going to cost you $580, an MSI 780 Ti on ebay is going for < $100. So it's not a matter of what you get performance wise, it's how much you get for the cost.
6. The big thing here is what you have for a current system MoBo, RAM, CPU and monitor (1080p). if it's an older X87, Z97 system, I think I'd go cheap and go the SLI option (assuming ya spend no more than $175 and your monitor is 1080p) ... if it's a Z270 system, I'd plop down more money for the 1080. And, of course, if I had a 1440 144 hz monitor, Id o for the 1080.