I had the 13" (non-touch) and now the 15", both 2016 models. With the 13", I really felt the need to connect to an external monitor to be my most productive. Granted, a second screen always helps with this, on the 13" it felt like a "need". With the 15", it would be nice to have but not a need.
For my daily work, I run Outlook 2016, Xcel, Skype for Business, multiple PDF's and multiple tabs in Safari. Frequent video conferencing, etc. as well with Skype. Since getting the 15", I've found it easier to get my work done and the screen is just big enough to have a Skype video meeting running and have Outlook or something else running next to it on the screen. With the 13", this was much tougher, but it could be done.
I mainly run the 15" at 1680x1050 (default res), but sometimes will bump it to 1920x1200. On the 13", the default res was 1440x900 which looks great. You can bump it to about 1680x1050 but then the default fonts get really too small for my eyes. I would do it sometimes and increase fonts in documents. On the 15", going to 1920x1200 also makes it smaller but I find it just a bit easier on the eyes. I use this less often on the 15" as well.
Travel: I still have my 12" 2016 MBr m5 that I'll be selling. I just had too many multi-tasking / power issues with it to be my main business Mac for daily use. For travel, it kicks butt! However, even on a plane, I took one flight where the seats were so narrow and the tray was half-sized, that I couldn't even use the 12"!! I just got back a week ago from a trip and took the 13" with me. It was a good travel companion. I certainly felt it more in my bag than the 2lb 12". I haven't traveled with the 15" yet. However, once in the bag, it seems to carry like the 13" did. You notice it, but it's not a problem. The 12" disappeared.
Overall, I chose power, productivity, screen space, and what is most likely a Mac that will see an extra year or two of use (with the quad core, GPU, etc.) vs. the 13". I figured that the mid-level 13" (no upgrades) would last about 3 years before I felt the need to upgrade to a more powerful machine, etc. For the 15", I figured the longevity to be about 4.5 years for me. The calculation below is my cost (with corp discount) and before tax and how much each machine would cost over the life of it for me out of the box (no accessories, screens, etc.). I also did the same for the m5 version of the 12" MBr. The 12", I figured that it would go for about 24 months before I felt the real need for more power, a second or third USB-C port, etc. Actually, I feel the need now

, but could stick it out for a while I think. Overall, strictly on numbers, the 15" made sense to me. Plus, it's increased productivity so far.
12" MBr: $1503/24=$62.60 (2016, 1.2 m5, 8G, 512GB, Intel HD 515 iGPU)
13" MBP: $1691/36 = $46.9/mo. (2016, 2.9 i5 dual, 8G, 256G, Intel Iris 550 iGPU)
15" MBP: $2255/54 = $41.7/mo. (2016, 2.6 i7 quad, 16G, 256, Intel Iris 540 iGPU + AMD 450 2G dGPU)
YMMV on what you value most. IMO, the 13" has a lot of value for travel and can be very productive. When I looked at moving up from my base non-touch 13" (power was really lacking for what I needed), I look hard at the mid-level or even higher level 13". By the time I upgraded one to 16 RAM, etc. it was getting closer and closer to the 15" price. When that happened, the price

erformance ration didn't add up for me. I went with the 15" and can carry it around easier than the old 2010 MBP 13" or even my old 2012 MBA 13". The old Air was "a dream" to travel with for years. It's my wife's Mac now. Holding them both, in hand, they feel about the size weight. Thickness is negligible and the overall size is just slightly bigger for the 15"....it's amazing what has been done here.
EDIT: one last thought. I also have a 2012 Mac Mini (i7 quad, 16G and a 240G SSD/1TB HDD and integrated graphics) that I'm going to part with. It has served me VERY well for about 4+ years. It's kept up really well, mainly (I think) because of the quad core processor and 16G RAM. Even with the old CPU, it's still going strong. I can sell it for CLOSE to what I bought it for in 2012 (stock, not upgraded aftermarket). The quad core is a big thing for me in the longevity and also pushed me to the 15". I was REALLY hoping for a 13" quad core. If that ever happens, I may change my mind