Updraft range hoods are mounted above the cooking surface and draw cooking smells and vapor up into the hood. They are the most common type of range hood and are often wall mounted or combined with a microwave above your cooktop.
Downdraft range hoods are integrated into the cooktop surface, and are nearly invisible unless it is a pop-up down draft unit which rises a few inches above your cooktop surface.
Updraft range hood blowers are usually located in the range hood, making the fan noise audible when they are operating. Down draft range hood blowers are located in the cabinet below the cooktop or in a remote location away from the cooking area.
A remote location blower will cut down the audible noise significantly when the range hood is in use.
Updraft range hoods have straightforward duct work requirements, and duct work is ideally routed straight through an exterior wall at the height of the range hood. Duct work for a downdraft range hood usually has a more circuitous path, especially if the down draft hood is located in a freestanding kitchen island.
Then the duct work has to be routed through cabinetry or floor structure to vent to the outside.
Range hoods that are wall mounted on an exterior wall can often be installed in an existing kitchen. Downdraft range hoods are more challenging to install in an existing kitchen, as the down draft blower and duct work require specific cabinetry design to accommodate the unit properly.
Ideally, down draft range hoods need to be accommodated for in a kitchen design from an early stage.
Updraft range hoods are located above the cooking surface, drawing cooking smells up and out of the cooking area directly. Down draft range hoods need to suck cooking vapor sideways or down to the cooking surface, requiring that the down draft blower be very powerful in comparison to the same volume of venting from an updraft range hood.
As updraft range hoods are more common than down draft range hoods, there is greater selection of inexpensive updraft range hoods. Besides the greater cost of the unit itself.
down draft range hoods are more expensive than updraft range hoods to install, due to special cabinetry requirements, labor and time to install them.