As long as you stay visible, you are normally safer on the street. The majority of car/bike accidents in this area are bikers on the sidewalk being hit by vehicles exiting driveways and parking lots. Intersections are also problematic on the sidewalk for a biker. If drivers check the sidewalk before they cross it, they don't look far enough to see a 15mph bike. A sidewalk with few intersections or driveways could be fine.