The lightbulb ones use conductive heating. Your weed just sits on the metal plate, which heats up and releases the vapors through the hose in the dome. The other ones use convective heating. In those, hot air passes over your weed to release the vapor. The general consensus is that the convective types are much better than the conductive.

Apparently the vapor genie is a good option if you want a small, portable vaporizer.
http://www.vaporgenie.com/default.asp