If you have to get a GPU right now, playing at 1440p, the Titan X might be more universally appealing since there is 0% chance of having SLI-related issues, but that's a hefty performance-to-dollar premium.
In many games, the SLI performance will blow a single Titan X out of the water. And at 1440p you'll basically never be limited by the 4GB of memory like some people convince themselves (remember you also have memory compression). But even as good as SLI is these days, there's no question that bigger single GPU is more reliably good--especially if you buy and play games just as they come out.
If you intend to play with a VR headset, then SLI undoubtedly gives better performance all around because of the VRDirect extensions.
Even if you are pro-NVidia, I would personally wait until mid/late June, after the new round of AMD cards come out. At least wait until June 3rd, when AMD releases the details about the new cards as facts to make a better guess about how pricing will be affected.
This is not confirmed, but it sounds extremely likely that the 390/390X cards will be a tweaked Hawaii but with the memory compression magic of Tonga.
Besides the 390/390X cards, AMD is also releasing their new high performance tier of Fiji XT which is going to compete with the Titan X, but it's rumored to be expensive as well (latest rumor is $850).
I personally buy from both AMD and NV and only buy based on performance/price for my use cases, but even if you only buy NV, the new release of cards will very likely affect/reduce the prices of the current NV cards.
That's my 2 cents.
Edit: In my recent interest in HBM, I've been more focused on researching AMD's new cards. I forgot that 980 Ti likely launches around the same time (there were very old rumors of a 3Q release), so that will be better perf-to-price than Titan X and all prices should reduce on older cards. The general 2 card vs 1 card stuff still applies as well as the lack of a memory limitation at that resolution. Regardless, we are too close to new releases to recommend buying right this minute.
Edited by peateargryphon - 5/19/15 at 5:53pm