I have a 17" and my girlfriend has a 15". We are both airline crew and travel with them overseas every week. I bought mine to replace my desktop and as an ex graphic design student, I really wanted the screen size. For watching DVD's in hotel rooms, and getting louder sound for movies and music, the 17" is well worth the larger size.
The downside is that if you travel economy class, the 17" can be too big, usually bigger than a tray table, giving you little room to be comfortable. Business class of course is no problem! As for lugging them around, they both need to be in a similarly sized bag, but the weight is noticeable in my opinion. For these reasons, I am considering buying the 15" when it is updated. But then again... if the 17" is released with features that tempt me, I may buy the 17". I tend to always want the flagship model!
At the end of the day, what is most important to you - screen size, speakers, and some other cool features, or a smaller, lighter alternative that is still an awesome machine?