I have the Acer X34p, which I bought a year ago. My decision was prety much between the same two you are wondering about. I bought the Acer for £900 (but was able to claim back the VAT, which was a significant amount) whilst the AW was I think £1,100.
Now, I have not used both I’m afraid, but I can say I am very very happy with the Acer. It was an upgrade from a 6 year old 27" Dell UltraSharp and I can’t see any loss in picture quality (UltraSharps are fantastic monitors). I left it at the default 100Hz, since at the amount of pixels, even my 1080 struggles to reach 100 FPS in most demanding games. Some do though and the G-Sync is great, as well as the increase from 60 to 100 Hz.
Tbh, I don’t think you can go wrong with either of them. I would just buy whichever is more handy to order. Just make sure the shop gives you a dead pixel insurance (I detest the idea of even a single dead pixel, when I am paying that much money).