4k on 27"?
1. Windows icons will be way too small.
2. If you are sitting around e.g. 3-5 feet away from the monitor, the benefit of having higher resolution on games is not really visible. You are wasting the benefits of having 4k, if the screen is too small. There are connections between screen size, screen resolution, pixel density and if you can tell the difference. 27" is just far from optimal for 4k.
More to PG279Q and PQ27AQ, PG279Q is a lot faster. It is a 144Hz monitor, which means you can go up to 144 fps on that monitor. PQ27AQ can only go 60Hz, which means 60 fps max.
You will really appreciate monitor with higher refresh rate, trust me! Especially for fast paced shooters like DOOM.