Over the years I have owned the following notebooks, 2006 - 13" MB (non-pro), 2008 - 15", 2012 - 13", 2013 - 15", 2015 - 13", 2016 - 13" and lastly 2016 - 15" and traveled with them frequently. Heres my take.
I love the 15" MBPs, you can tell by my track record that I will bounce around, but I always tend to come back to the 15". This is mainly because I work in IT and I am used to having a 3-4 monitor workspace, so when I am traveling or going site-to-site, I need the max amount of screen space I can get, because my day is non-stop multitasking. I made a 13" work but was never fully satisfied with the amount screen real estate I had to work with. I found myself doing things on my iPad Pro that I would typically have done on the laptop because I had the same amount of usability with it as the 13". I had originally pre-ordered the 13" 2016 tbMBP, I was thoroughly enjoying it, but I couldn't get the possibilities of the 15" off my mind. I was constantly questioning if I REALLY should have bought the 15" instead. I used that one for close to a month or so had an issue with the "O" key on the keyboard sticking and used the opportunity to return it and buy the 15", I will say I have been satisfied with my decision and do not have "buyer's remorse" looking back.
I love the 13" for different reasons, it is extremely portable and has been since the Retina models were introduced. The 2016 model is even more so, for God's sake the thing is just barely larger than a 12.9" iPad Pro and its a full fledge notebook. That's impressive. I will say, I got WORLDS better battery life out of the 2016 13" than I am getting out of the 2016 15". I'm talking HOURS better battery life and I am doing the exact same crap on the 15".
My truthfully deciding factors were these:
- Power
- Screen real estate
- Already carry a 12.9" iPad Pro, I didn't need the same exact thing with a different OS. (Basically)
Thus, my bag is filled with the 12.9" iPad Pro w/ KB and 2016 15" MBP.
My advice to you would be this:
If you ACTUALLY need the power and/or screen size of the 15" and battery life isn't a big deal to you, then go for it. But if portability, battery life, and moderate power is what you are looking for, I'd go 13".
Just like these guys have said, if you think you'd never get the other off your mind, then go buy one and if you don't fall in love, take that sucker back and get the other one.