I decided today to go with the Knog Pig Dog 15.5" bag. It is basically a messenger bag with a rack attachment rather than a pannier that has a strap to carry it. I figure that I'm only really commuting 2 days a week at most and the rest of the time I'm taking the subway. That plus I need to use my bag during the day at the office. It made more sense than a pannier. They're sized for Mac Books, but I carry a dorky Lenovo. Price was right too. If I find it too small, which I doubt, I'll supplement it with another bag, maybe the Dry Dog, which is more of a pannier.