Do you really think they are close? I guess if you just compare price, they are close. The specs the monitors have are very different. The Acer Xb280hk is a 4K, 60hz, G-sync, with a TN panel $800. The soon to be released Acer Xb270hu is a 1440p, 144hz, G-sync, with a IPS panel, also $800.
Sorry for comparing Acer and G-snyc. I guess your for AMD, and want Free-sync. Then you can get the, also soon to be released Acer Xg270hu, 1440p, 144hz, Free-sync, with TN panel $500. (these are US prices)
I would get a 1440p monitor, over a 4K. Have you ever been burned by trying dual GPU's on a game and only a single GPU works? That has happened to me because of driver support.