Although I'm aware that this is a thread which was begun in 2012, since at the moment I actually do have one of each size -- 13.3" rMBP and 15.5" rMBP I can offer my anecdotal experiences, too. I initially purchased the 13.3" rMBP back in 2013 to replace my first-generation MBA and I found, when viewing specs and such, that an rMBP would suit my needs more than another MBA would have done, plus the rMBP had the retina screen; as someone else said I felt I was getting more bang for my buck. I found that I loved the rMBP so much not just for its portability and flexibility in use but also because with the SSD rather than a "spinner" HD it ran circles around my 2012 iMac. I decided that when Apple brought out a retina version of the 21.5" iMac that I'd go for that, too, with a speedy SSD and such (the 27" iMac is just too darned big for me, the space I have available, and my needs). So Apple finally released the 21.5 iMac with retina screen -- and the specs were disappointing. SSD was not standard, for one thing. I compared the tech specs with those on the versions of the 15" rMBP and realized that in the long run I'd be better off simply buying a 15.5" rMBP that already had the specs I wanted and the costs would be about the same by the time I had Apple put together a BTO iMac with retina screen and ship it to me. Faster, too -- walked into the local Apple store, bought my 15.5" rMBP, brought it home and that was that.
I had been using the 13.3" as a desktop replacement some of the time by positioning it on a TwelveSouth stand but had found that the screen real estate was just a little too small, especially when watching movies or doing other things. The SSD made it so much faster than the 2012 iMac with its 5400 rpm spinner that I found myself neglecting the iMac and using the 13.3" rMBP instead. That machine is great for traveling, too, and it has gone with me on trips with no issues whatsoever. I also will be using it in the bedroom more now, positioned on my lap while I sit in a chair, feet propped up on the bed. I've never tried lying in bed or on a sofa with any computer on my chest -- instead, I use the iPad Air 2 when needed in situations like that.
I'm retired so don't commute to work everyday, with or without a laptop in hand, but definitely the 13.3" is lighter in weight than the 15.5" rMBP and that is the machine I prefer to carry with me when traveling or even just going over to a friend's house and needing a computer with me. Portability? 13.3" wins hands down.
As a desktop replacement machine, the rMBP wins -- the larger screen real estate provides better viewing and the discrete graphics available also make a difference as well when doing graphics-intensive tasks. There is flexibility in the knowledge that if I decide I do want a larger screen at some point it will be simple enough to pick up an external monitor somewhere. The two USB 3.0 ports and the two Thunderbolt ports plus the SD card slot are handy to have, although since I'm pretty much wireless everything (keyboard, trackpad, mouse and printer) unless I plug in an external drive for backup purposes I don't really need to have anything attached to this machine. At some point I may want to add a more-or-less permanently plugged-in external drive to add more space if that starts to become an issue.
In the end each of us needs to assess our priorities and how we will be using a computer and what features and functionalities are more important. After I finish setting up my new 15" rMBP I will be selling the 2012 iMac and this will be an all-portable household when it comes to computers. The 15.5" rMBP offers the kind of flexibility and functionality that most meets my current needs by being useable as a desktop replacement in the main living area while also still being portable enough to take with me somewhere if the need arises. The 13.3" rMBP offers real portability and will be the machine that travels with me on trips and around to friends' homes as needed, as well as serving as the secondary computer that lives in the bedroom and that I use at night.
I know that this is a hard decision and it would be a matter of weighing the pros and cons, the benefits that each size laptop has to offer and taking into consideration the ways one would be using the machine on a day-to-day basis as well as for situations such as traveling.