As far as reliability goes, Jem (Martin) are about the best. Some of the smaller models eg Magnum and Technofog are available very reasonably now, though the ZR series are the type most installers use. They also make machines for other brands.
The cheaper chinese ones (and anything under about 80 quid!) may seem good value but they tend to suffer from scaling up especially if they dont see much use; this is when limescale solidifies due to evaporation. The cheap ones have small heater, small pump and small bore pipes....opposite for the better ones.
Having said that, the cheap ones may be considered disposeable, though a little inconvenient if you only discover they're duff at a venue.
And dont skimp on fluid! They're best used with the manufacturer's own fluid..esp. the Jem machines.