Open sounding heads are like loud, ringy, clear and musical.
Closed sounds are those heads with some form of muffling which chokes the sound some. Heads with coatings, tone rings, etc, will reduce overetones and help punchiness but not necessarily projection unless miked up or played hard.
I think it better to buy heads to suit the style of music you play rather than a head choice that you have to muck around muffling with in whatever form you use.
The site below sort of explains, always consider that a head is the biggest factor to any drum sound and that the sound coming out front of the kit is different to what you hear from the throne.
http://en.wikipedia.org/wiki/Sound_of_a_drum
Hope of some help?