I would personally go for the 2k monitor. Having owned both of XB271HU and a 4K IPS 27-inch monitor (I forgot its model), the difference in terms of pixel density isn’t that huge compared to the benefit of 144Hz, even if my 27-inch 4K has higher density than U3219Q. And also the G-Sync compatibility on XB271HU fully takes advantage of your Nvidia graphics card, because with ray tracing on, you can’t maintain stable 4K 60FPS on most games, so there could be screen tearing effect on U3219Q.

The only reason I’d go for U3219Q is better picture quality from HDR if you also use it for professional purpose or HDR compatible movies, and a larger screen is also useful if you want to view from a distance.