I dont have any stats to back it up but I travel through ORD a lot and the winter is better for me than the summer. I get far more delays in the summer (storms) than in the winter but the big difference is you never know when you are going to get that big snowstorm that shuts the whole airport down for a day and screws everything up. Those news stories sure make you think the winter is the worst time to fly through ORD but winter delays have not been any worse than the summer ones.