It's not a huge difference, but according to this calorie calculator, a 150lb person will burn 119 calories(with selecting what I think is a commuting pace) in 15 minutes. Selecting a moderate walking pace it's 156 calories in an hour.
http://calorielab.com/burned/
When I walk home I feel like the effort was a bit more than biking, since I'm at it for longer. But maybe that's because I bike so much I find the effort lighter than most.