I went through similar testing and dilemma as you did. Except my decision was to go for the 13" MBP.
For me, using the 15" prior to that I didn't like it, I felt for the screen real estate you gain, which vertically is about 3/4" it wasn't worth the extra bulk and weight. I mean just not weight in a bag but rather how cumbersome the 15" is to use on a daily basis over a 13"...
I feel a 13" can't be beat when away from a desk, on your lap, couch, cafe, bed, back yard etc the 13" is more 'handy' and 'fun' to use in these circumstances, for example, type on a 15" MacBook Pro on your lap for an hour, then switch over and type on a 13" on your lap, do the same at a cafe, outside etc etc.... I think most in these cases will for sure choose the 13".
At a desk it doesn't matter, that's where 15 or even 17" notebook screens even seem small because most of us are used to 20, 22, or 24" monitors on the desk.
So for me, the 13" was the best blend of power and portability, I may miss 3/4" of vertical screen space very rarely, but I'll never miss how convenient it is to bring and use everywhere.