In all honesty, I sort of skimmed the graphics portion of the ASUS X99 specs and the absence of onboard graphics just didn't sink in. I was more interested in the M.2 treatment.
But like the Z97 ASUS the X99 only supports PCIE, not the SATA SSD which puts us back to hunting for or waiting for a PCIE 3.4 drive. Or as you seemed to indicate, using the Samsung 850 Pro as an interim O/S drive until.... If that turns out to be the plan, for now, the fact that the ASRock X99 offers support for the M.2 SATA is irrelevant, I think. I will look at the proper ASRock board you linked just for giggles.
Edit: The ASRock reviews seem pretty split but mostly around OCing which is of no interest to you, or me. But the ASRock customer service responses didn't sound very encouraging. The fact that the OCing tuning software is shoddy and the BIOS are not great, suggests your comment earlier that ASUS is more 'polished' is probably correct.
If this were me, I think I'd stick to ASUS.
Edit: I've reviewed 9 pages of i7 5820k comments and, of those that mentioned M/B s, ASUS was about 3 to 1 over ASRock.
Fractal Design R5 | Asus Z170 Pro | Intel i5 6600k | 16 GB G.Skill Ripjaws DDR4 2133 | Seasonic 650w PSU | eVGA GTX 550 TI | Samsung 960 M2 500 GB | Samsung 850 EVO 500 GB | ASUS Burner | Windows 7 64-bit