At the time of CEX, ASRock had no plans for AM4 ITX.Thought so. Didn't think ASROCK would pass up AM4 ITX. So that makes Biostar, Gigabyte, and ASROCK; the three we heard about during Comdex. That's disappointing ASUS. Would have loved to see an AM4 ITX board from you. If MSI releases one before ASUS I would be really surprised.
same for the B350GTN:
On the B350 board, there is a quick switch between two of the SATA headers and the M.2 slot. So you will be able to run it as either x4 M.2 and up to two SATA devices or x2 M.2 and up to 4 SATA devices. The X370 board will be able to support x4 M.2 and up to 4 SATA devices all at once.
There are also different USB port numbers, but I don't think enough are exposed to begin with to make any difference. Someone was checking if the X370 boards x16 PCI-e slot could be bifurcated, but I haven't heard if Biostar got back to them with an answer; the B350 would not be able to be bifurcated.