I found a local camera store -- Kenmore Camera of Kenmore, WA -- that carries both Think Tank and Gura Gear bags. They already had the Urban Disguise 50 and 60 in stock, and ordered a Chobe so that I could compare them all.

Both bags met most of my criteria: they have padded camera sections, room for camera accessories, room for my laptop and accessories, handle sleeves to fit on rolling carry-ons, and they're both expandable.

However, the Urban Disguise's expandability is to allow the camera compartment to grow at the top to hold larger gear. Its footprint doesn't change when it's expanded or unexpanded. The Chobe, on the other hand, expands and shrinks as a unit, so its footprint shrinks and grows. The net effect is that while I was sure I could fit the Chobe under an airline seat in its unexpanded state, I had no such confidence with the Urban Disguise 50, and was sure the 60 wouldn't fit.

I was also happier with the size of the Chobe's laptop and tablet compartments.

Neither bag was obviously weather-resistant.

I ended up choosing the Chobe. It seemed like a better piece of kit all around. When I'm traveling with all my eventual camera gear -- NEX-6, four lenses, flash, power adapter + cable, etc. -- I can expand it and store it in the overhead space. When I'm traveling lighter, I can contract it and store it under my seat, meaning I can put my rolling carry-on overhead and not have to check a bag.

I hope someone out there finds this useful!