I recently went away on tour with my 17" MBP and also took a 15" as a back up, the difference in "portability" is nothing much, both still have to be in a laptop bag, and as I had 2 bags the same, buy the time you also put in power supplies/mice and whatever I didnt really know which was which until I opened them.
of course the 17" screen is much nicer to work with.
