I stay off the sidewalks whenever possible because pedestrians are far less predictable than cars. They stumble around randomly, never look up from the sidewalk, flail their arms about with no warning, and exit buildings without looking both ways first. Not to mention cars coming from driveways aren't looking for bikes when they cross the sidewalk.
On streets with lots of fast traffic though, a sidewalk with no pedestrians is probably safer. Unfortunately, those are usually the broken-up, uneven sidewalks covered with broken glass. :-/