Hi,
The best Canon autofocus motors are the ultrasonic (USM) ring type. These are identified by a gold ring on the prosumer lenses such as the 85/1.8, 50/1.4, 100/2.8 macro, etc. They're also on all the red-ringed L lenses. The chief advantages are that USM is fast and quiet and allows manual focusing even when the lens is set to auto focus.
Some of the consumer lenses in the 28, 35, 50 focal lengths use an older autofocus technology that is rather clunky and noisy, not a big hindrance for landscapes but a disadvantage for sports and other fast-moving situations. You can also inadvertantly damage one of these lenses by manually focusing it in in autofocus mode.
Hope that helps a little.