Well the volcano is a fog machine and the haze generator is a haze machine. Haze is mush less dense than fog and can be run pretty much continuously without much notice. Haze creates a very thin "film" in th air which allows you to see the beams of light from your lights. Fog however is more of a burst effect that gives you that "wow" smoke effect. Which of course allows you to see the beams. Eventually is dicipates and then you burst it again. Haze you can let run. Ive also never heard of a fazer? It depends on what kind of effect you wanna go for...the more "wow smoke burst effect" or the more "subtle hazy effect."
-AMP