The EVGA Hybrid version of the Titan X is pre-assembled. All you have to do is install it, and then you're good to go. There's no messing around with coolant or anything because the loop is totally sealed. You just need 120 mm spots with about 2in+ of space to fit the 120mm radiator and fan. As long as it fits and has enough power, it will work - no fuss.
People are saying the 980 ti runs faster because MSI, Asus, EVGA, etc. have permission from Nvidia to overclock the cards fairly high. That just makes the core components run a little faster, but that doesn't make up for the fact that the Titan X has slightly better core components. If you run a Titan X at the same speed, it will definitely be faster, and that is what the Hybrid version does.
As for SLI, it works very well on high end cards like the 980 ti and Titan X. But you're right. It does have issues at release of games. There's usually a couple month lag where drivers won't support SLI in brnad new games, so you'll run just for card if you buy games on launch day. But that almost always clears up in time, and by that time, the game costs less anyway.
About VRAM, there's no telling now what will be "enough" in the future. Expect the lifespan of these cards to be about 3 years. By "lifespan" I mean how long they will keep you toward the very top end of computer gaming, not how long they will actually last. These cards should last longer than 3 years.
Anyway, the point is that you have to guess about whether 6 GB of VRAM will be enough for the resolution you want to run in three years. I'd guess that it probably will be enough - probably. The price of not having to worry about VRAM at all is the difference between the 980 ti and the Titan X, since 12 GB of VRAM will be more than enough more any game. That amount of VRAM is really intended for computational computer. If you're like some of my friends who do computer science research or run GPU-accelerated compilers for code, then you'll use the 12 GB more often than a game (and a gamer might not ever use that much before the card becomes obsolete).
All that said, two 980 ti cards in SLI would be about 85% - 95% faster than a single Titan X (i.e. almost twice as fast). The first of two exceptions will be for when you're waiting for an SLI driver to come out. That problem largely resolves itself, though. You'll be able to run most games on just a single card anyway, so when the update comes out, it will be just a bonus. For games that you'll need both cards, you should just wait until the SLI update comes out - and buy the game at a lower price. Then the second exception is older games that never released an SLI profile. That's a non-issue for a Titan X or a 980 ti. Either will run older games maxed out just fine, even at higher resolutions. There may be other exceptions for when SLI won't work, but I can't think of them. By and large, SLI works very well on flagship cards.
With $1,500, you could just get two 980 ti Hybrid cards right now and then put the other $1,500 in a few months toward a really sick monitor. If you get a Gsync monitor that kicks ass, then running at 40 fps is just as smooth as running at 60 fps on a standard monitor because things don't fall out of sync and stutter. That would allow 4K on an SLI setup with pretty smooth play just about all the time, and the monitor will last you longer than any component in your entire computer.
There are really good 1440p Gsync monitors, but they also make them at ultrawide (3440 x 1440), and 4K. Acer makes some good ones, and Asus just announced a new ROG Swift that's supposed to be excellent. Whatever you do, just be sure to get an IPS (or non-TN) panel. You have too much nice equipment to be on a TN panel. The tradeoffs are now minimal compares to the vastly superior image quality on an IPS display compared to a TN. Getting into OLEG is another ballgame, but those won't be available for computer monitors with Gsync for years.