The benefit of Freesync/G-Sync diminishes if you are pushing 100+ fps, compared to ULMB or even just basic operation of 144hz. Unlike the ROG Swift, the XB270HU's response time is linked to refresh rate, so the higher the frames you can get, the better your response time as well.
I will admit the XB270HU was fantastic, but unfortunately it had dust behind the screen and dead pixels so it was returned. I have been waiting since the ROG Swift's announcement for an IPS variant of the 2560x1440/144hz/Gsync(/Freesync) combination, and am hoping the PG279Q has decent QC out the gate if Acer doesn't sort things out.
Agreed, both G-Sync and FreeSync aren't that good at 100+ FPS, where they are good is in the 30/40-75 range and lower.
The low end is where G-Sync beats FreeSync pretty handily. Both my G-Sync monitors are 30-144Hz, most of the FreeSync are 40+ Hz.
On 4K monitors with VRR, you are much more likely to see lower FPS, and thus you need a better lower end VRR, which means G-Sync.
Having said all that, I see NOTHING to make me want to sell 2 monitors and a video card and "switch teams". AMD will have to do a LOT better job at multiple things to get people like me to bother to switch.