[selden]

Dipoles are supposed to give a more dispersed localization, since their null points toward the listening position so the sound you hear is reflected off other surfaces. Monopole speakers give more localization, since you hear the sound directly from them. Which is better depends really on your personal preferences -- ambient sounds or directional helicopter flyovers

If the processor includes support for width speakers, supposedly monopoles have an advantage in that situation. I've read claims that width causes more of an improvement to the soundstage than rear or height.