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I just got back from sitting in economy for about 20 hours on a plane. The MBP works alright, but it's a tight fit. If the person in front of you isn't reclined, it's fine. If they are (and most people shoot their chair back as soon as the plane is in the air), you're going to be pulling your arms in pretty tight and balancing the laptop kind of precariously. It can be done, but it's not all that much fun. That said, the 13" whitebook doesn't provide that much more flexibility, either. I'd stick with the MBP, as it's more versatile when you're not in a plane (which is probably most of the time).
 
I carry my 15" around with my everywhere, school, coffee shops, planes, cars, buses, the couch and its never really been a problem with the size. And like others have said the screen is amazing over the macbook. Plus i needed the extra power but for portability its about the same as my sisters uni-macbook. So i say keep it and never look back. :)
 
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