Getting doored into traffic is way more dangerous than getting doored into a sidewalk. If you have data that suggest otherwise, I will back down on this point, because we ought to make these decisions based on the best available evidence. But the way I see it, the best available evidence would say sidewalk-side is better. For example, the wikipedia article on dooring says "[G]etting doored itself usually is not fatal; rather, most serious door-zone-related injuries are sustained by getting hit by a motor vehicle after colliding with or swerving to avoid the obstructing door. Thus, most dooring deaths and serious injuries occur in the travel lane and not in the door zone."