I have an older 13" MBP and have just been considering a new laptop. A big internal debate for me was whether to go 13" or 15" this time around (I've always had 13-ers). I'm not a super-power-user, so either size can be built up to more power than I need - so that's not a factor for me (can't tell if it is for you or not).
As I get older my eyes are not as good as they used to be, and wearing glasses while using the computer is a pain (it's at an awkward distance, not close like a book, not "far"). So for me it comes down to two "opposites":
1) 15" has nice, large screen. Ahhhh.
2) 13" has nicer (for me) form factor. And it just seems more "efficient" with the keyboard just fitting the base and etc.
Now, the current 15" is the same weight as my older 13", so I know I can handle the weight, but.... I like to use the computer on the couch, propped up in bed, etc. I also will pick it up and carry it around, use it in the car, on small tables, etc. For those things I just feel the 15" might be a bit "bulky." Those small differences in size kind of double when you open the computer, if that makes sense. Is it doable? Sure. But.... as much as I would love that larger screen, I think the 13" is probably more me. It's the form factor.
I really want an 11" with a fold out screen
I actually was so unsure between the two that I decided to soup up my current 13" with an SSD and wait a while to buy a new one, but I figured I'd type out my thoughts in case they would be useful.