The M20 and M21 have proven themselves to be amazing lights. The new upgraded LED makes an already great light even better. The XM-L LED allows both lights to have more output while also being more efficient. Not only will both lights be brighter than the previous versions, they will also run longer. The user interface is essentially the same as the previous versions but there have been improvements. Both lights still use the dual button system which has become widely popular in this class of lights.
The M20 and M21 are the epitome of the tactical light. Both will work well as a duty light, weapon light, or just an every day companion.