It would help to know what your intended use is for the card, but...
Titan Xp is the fastest for gaming and rendering, but the abilities are marginal compared to the 1080 Ti.
Firstly, an overclocked 1080 Ti will perform more or less the same as the new Titan Xp, stock.
Secondly, the 1080 Ti's are roughly 40% cheaper than the Titan Xp.
Thirdly, The cooling on an overclocked 1080 Ti aftermarket version will be MUCH better than the reference card of either the 1080 Ti or the Titan Xp.
There are multiple reasons to go with a 1080 Ti over the Titan Xp, the only reason to go with the Titan is if you are doing some seriously hard core rendering that you want the absolute most you can get, AND you have a huge budget. We're talking $450 for maybe up to 10% more performance in rendering only.
There are no hybrid versions of the Titan, you can only get it from Nvidia directly (or third party sellers who will likely gouge the price) you would also need to buy a custom cooling bracket and a loop, which would easily be another $300. So you're looking at $1,500 for a single, and for a custom loop about $3,000.
In my experience, the 1080 Ti cools just fine on air, has plenty of options for hybrid version (though they aren't always great) and is much much cheaper.
There are also those people that will inevitably argue that AMD Vega is coming soon and you should wait... however, AMD has always been overhyped and underwhelming on release... they said the same thing about the 480 being able to compete with a 1080 but we all saw how that's going. At most they are going to come up with something that will be somewhere between a 1080 Ti and a 1080, the most definitely will not be able to keep their claim of "blowing the 1080 Ti out of the water." Either way by the time those come out the next gen of Nvidia cards will be announced and will firmly take control of the high-end GPU market once again...