If you really need portability (that means carry around your rMBP every day) the 13" is your choice.
If you instead plan to use your notebook mostly at home, then the 15" has obviously a better screen (it's really amazing to look at a 15" rMBP).
If for any reason you need extremely high graphic performances because, for example, you work daily with 3D modelling, then you must buy the 15" with dGPU (that will not probably be available in the base 15" but only as a BTO option) regardless of portability.
If money are important, the 13" is way cheaper.
If you have or want to buy a Thunderbolt display (a new 4k retina version will probably be announced next tuesday or, if not, will come out very soon) again go for the 13" given it's better portability and the fact that you will use the big screen at home.
Out of some very specific needs (that basically can be summarized with "lack of dGPU) I don't think that differences in CPUs between 13" and 15" are so relevant for almost all users. However if you need to use many (x>2) virtual machines and/or more than 3 CPU-intensive programs at the same time, you will enjoy to have 8 virtual core instead of 4; so go for quad-core in the 15".
I hope this helps you. In my opinion the 15" must be considered to be the real professional machine, while the 13" is something in between a purely consumer device (the Air) and a real Pro one. For professional I mean a laptop that can be productively used by people who works with programs that requires huge graphic (15" dGPU only) and CPU raw power. Examples are 3D modelling, videomaker, professional photographer that elaborate say 100+ raw files every day, people who works with big database etc.. Of course nothing forbid you to buy a 15" rMBP just for facebook (to enjoy its screen) but you will be more than fine also with the 13".
I think anybody who is undecided between the two screen sizes should just go into the Apple store and do some comparing. That's what I did. It's really not a hard decision.
But you summed everything up nicely. I just think the size of the 15 is way too big, where as the 13-inch is perfect.