Problem is both have same resolution and I think 21.5 have more ppi.. He wants to buy the 24" one but i have heard that bigger monitor will cause to see the gap between pixels and image would be not great as a small screen.. Will 24" have this problem? He use it to gaming.. Also 23" monitors are not an option..
More aboutchoosing monitor
the view sonic is a great monitor i am using a similar on to the one you posted there great monitors. so i would go with the view sonic and plus i never even herd of Benq and are you buying from the site that you link the monitors to?
go for samsung led moniter it have better pixel rate contrast ratio and also support hdmi dvi for mutiple display