Good old GPUBoss offers a direct comparison summary. While some of their scoring methods seem somewhat mysterious and arbitrary, the conclusions they draw from their scores are quite decisive.

Titan X is a superior GPU on any fps game or benchmark, no contest. Mostly because it has a very phat 12GB of GDDR5.

Titan Black has fully-enabled double-precision FP capability. If you do any GPGPU Compute, CUDA, simulation, mining, or other such stuff then you already know this capability matters for your application far more than any number of fps.

Personally, I view the Titan X as being more sort of like a "980Ti Black", and the lack of FPDP means that in the long run it'll price out more like a 780Ti than like a true Titan. Money may be no object, but I wouldn't advise expecting it to retain it's resale value a few years down the road like all of its Titan predecessors.

And personally, I would wait until Q3/2015. AMD will roll out its full Rx-300 Series product line. NVidia is expected to release a 980Ti and to "surprise" us with yet another new extreme-tier product. Even if you aren't especially interested in these new GPUs, you can still take advantage of the Titan X (along with all other existing GPUs) shuffling down a spot on the pricing hierarchy. Another bonus - a few newer and better factory-overclocked nonreference Titan X cards might be available by then, since NVidia will have shifted its focus onto their newest new baby and allow the NVidia Partners to do their own thing.