Thanks for advices,
I accidentally went today in a radio technicians room and was able to listen the difference between mono and stereo sound. The technician explained that you can feel the difference only when you record sound from more than one sources which is usually the case in music orchestras which are not my primary video target. He also said that the microphones on digital cameras are poor sound recorders which is another reason why mono sound movies are more and less the same as in stereo ones.

So, am thinking to go for HD mono video....