I have a 17" and travel a lot by car. I really like the real estate on the 17", so personally I would always go with a 17". My 12" iBook G4 was smaller and easier to carry around, but I always hated the tiny screen.
I have a Brenthaven Pro 17 (no longer made) bag which I carry everywhere. The good--and the bad--thing is that it holds a lot--all the accessories, a portable hard drive, my camera charger, my cell phone charger, a cable lock, lots of cables (always carry an ethernet for hotel rooms). It's great, but a lot to lug around.
I spent a long time trying to decide between the 15" and the 17", and in the end I opted for the bigger screen and have never regretted it. It's on my lap right now (I know I'm not supposed to), an it rests comfortably on my knees. I've never had a heat problem--mostly feels about like a kitty cat. It's perfectly comfortable to type on for me.
Spend some time at the Apple Store or Apple retailer, and try the different screen sizes and see what feels best to you. I would also check with the bag manufacturer about whether it would fit under the airline seat. Last time I flew, I didn't take it, but I did measure the space under the seat in front of me, and my Brenthaven bag would have fit with no problem.
Ultimately it will boil down to personal choice--whether you like the larger screen enough to put up with lugging around the extra weight.
Good luck, and happy hunting!