On the continuous feed machines there are two reservoirs for water. One is for filling and the water does not get hot. The other is for the steam and letting excess water out. The ability to refill water continuously is more important than you might think. This can be important when you want to avoid the splattering of dirt particulates all over the place.
Some items require a more dispersed steam - large areas, plant misting, upholstery.
The lower priced units either do not have this feature or have a dial control on the canister. Length of warranty on boiler and other parts. It is usually 3 years on a good residential vapor steam cleaner and 5 years on a commercial vapor steam cleaner. Some units have a lower price because they only come with a one year warranty. Be sure to check this out.