Thanks to DisplayPort 1.2's bandwidth being limited to a maximum of 17.28Gbit/s, shoppers looking for a high end variable refresh rate gaming monitor have a tough choice to make.  Leave aside aspect ratio, colour depth and panel type for the immediate question; do you prefer the higher definition of a 4K display but with a limited maximum refresh rate or will you be satisfied by 1440p or 1080p with a refresh rate that can hit upwards of 200Hz?  The Predator XB321HK chooses path of greater resolution, offering 3840×2160 but with a maximum refresh rate of 60Hz, on an IPS screen with 4ms grey to grey response time.  If you prefer an MVA ultra-widescreen with a higher resolution, perhaps investigate the Acer Z35, if the XB321HK is closer to what you are looking for check Hardware Canucks full review here.

"With a sensible 4K form factor, a G-SYNC module and a stunning IPS panel, Acer's Predator XB321HK is the stuff gaming monitor dreams are made of. Unfortunately its refresh rate is limited by today's interface technology."

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