It looks like that LG screen is 1440p@144Hz with an IPS panel while the Asus is 4K@60Hz with a TN panel. A 5700 XT should run better at 1440p, and it's probably good to have access to a 144Hz refresh rate for games that can push well over 60fps. Plus, an IPS screen should provide better colors and viewing angles than TN. So of the two, the LG screen is probably the best option. Of course, being used, you might want to verify that it's in proper working order and doesn't have any issues.