This depends on what GPU you have and what kind of experience you want.
The Acer XB281HK is the obvious superior monitor, but 'Is it worth it?" lets take a look...
- With a 1ms responce time Vs the 5ms the LG monitor has, this will barely be noticeable but is still in the Acers favor.
- The LG is an ISP screen which means the colors will be more faded and less beautiful than those of the Acer's TN screen. This doesnt affect performance but is a pretty important factor if youre going for 4k because you want it as beautiful as possible. Acer is far superior in quality.
-The LG sports Freesync technology which is meant for AMD GPUs, where as the ACER has G-sync which is meant for NVIDIA GPUs. Depending on what GPU you have you should pair it with the appropriate monitor.
- The Acer is exactly 1" bigger which isnt a performance boost but is worth noting.
There are other differences between these two monitors but none worth noting (both 60hz, max tallness, etc.)
TL: DR- If you have an AMD GPU, I recommend the Acer monitor anyway but it would justify buying the LG one.
If you have an NVIDIA GPU then there is no reason to buy the LG.
Hope this helps, good luck.