If you have a duct system, above all, be sure the ventilator is ducted outside. Ductless (or recirculating) range hoods filter soot and some odors emitted from cooktops, but most pollutants are recirculated right back into the room. Ductless hoods aren't recommended with gas ranges.
Ducted range hoods use fans or blowers. Propeller-style fans twist air up the exhaust duct, causing the air to drag against its sides and reducing the exhaust output. More efficient blowers suck air without causing as much turbulence, reducing air drag inside the duct.
To choose a hood, measure the cooking surface to make sure the hood will cover all the burners. At the store, watch hoods in action. Look for a design that pulls rising cooking vapors and contains them until the blower can carry them outside.