This is really a matter of opinion.
I have the 13" MB, and would never go for a 13" again.... it's just too small!
The 15" is a great solution, as it's still highly portable, yet a reasonable viewing size.
I don't know how used to the 15" inch you are, but I would certainly spend some time in an Apple store playing with a 13", or if you have a friend with one, using theirs for a while to help you with your decision!