The fundamental difference between the Acer X34 and ASUS PG 35 is the price. However, ASUS PG 35 is armed with an amazing gaming monitor so it is worth the spending money..
If you are a gaming buff then it is an apt choice for you, the refresh rate is 200 Hz and the display is of 35 inches. The stand, build quality & housing is much better in ASUS PG35.
It is definitely worth upgrading for the same.