The only discerning difference you need to know is that:
X1D is 16bit while the GFX is 14bit a difference of well.. [281-trillion] no.. not a typo. Its what you pay premium dollars for. that added cost and performance comes from the Graphic Engine and has nothing to do with the sensor per se. Imagine if you will a PC's Graphic Card, the higher the performance the more costly it is.
That said, I believe that the GFX will be the system to beat for at least the next 3 yrs, before Fuji drops the GFX100s if not sooner.
BTW no one in their right mind here should be mocking Fujifilm in the least bit, or belittling what this Mega, Giant, behemoth, Corporation, that won't be in the least bit affected by the sale results or failure of the GFX system. I mean do I really need to remind anyone of how important they are to the photographic industry and their contribution: Broadcast - Cinema - Film [yes 135's and instant film till today] - Digital - Fabrication Facilities - Largest Lens manufacturer in the world period for every conceivable industry.
In reality and the real benchmark when it comes to GFX particular Sensor size and performance level would be the Pentax 645z (mirrorless aside). A true trailblazer that deserves more respect than its been given.