The MX has a slightly faster response time and would be better for gaming. Is there a particular reason you're buying into an IPS panel? Are you interested in the brightness, color accuracy, wide viewing angle, etc. for purposes other than games? Just curious. If you don't necessarily need all of that, there are plenty of other ASUS monitors in that price range with 1 or 2 ms response times that are better suited for gaming purposes.
Honestly the one that looks the most visually pleasing. If the PB238Q is reportedly better in terms of ghosting and speed, then go with that one. The 1ms difference won't be noticeable unless you have it side by side with a much faster response time screen.