The "HM" model is an upgraded version. This from their website:
"Flicker-free Backlight for Visual Pleasure
The GW2255HM is upgraded to eliminate flickering at all brightness levels, effectively reducing eye fatigue"
Full specs here:
GW2255HM: http://www.benq.com/product/monitor/gw2255hm/
GW2255: http://www.benq.com/product/monitor/gw2255/
Print off the two spec sheets and you'll be able to compare every detail for yourself.