I've owned both the PG279Q (What's the BLB on yours like?) and the Acer X34a. The only games I had issues with were some newer Telltale games, and watching films with black bars either side was annoying but Netflix has a Chrome extension to fix that. Other than that the gaming experience was amazing, and if a decent HDR ultrawide comes out i'll probably buy it.
I missed the extra Hz you get from the Asus but not sure this will be an issue with the Alienware.
What monitor do you have now? Do you need to update your sig?