The Shimian is neither "better" nor "inferior" to your Sammy 120hz monitor.
Whether changing into a 1440p monitor is an upgrade really depends on your preferences.
In terms of monitor real estate the Shimian will definitely be an upgrade from your Sammy. However, for the extra real estate you trade away the "smoothness" of your 120hz monitor, you will start to see screen tearing when playing games (especially fast paced ones).
The Shimian displays colors better than the Sammy, although you might need to recalibrate the Shimian for it to be close to color reproduction accuracy as the premium models.
So, if you're the type who wants beautiful colors and vivid images and are willing to trade screen fluidity for it then the Shimian can be considered an upgrade for you.
As for green-sum, he's had mixed feedbacks on him, some are good while others are real horror stories, read the Achieva Shimian Monitor Club, TA_Planet, accessorieswhole, and dcsamsungmall seem to be the most with positive feedback.
I know some people here will start recommending the Yamakasi Catleap 2B model but those monitors are very rare, the other site that sells these monitors has a long queue of orders.