In the same boat. Carried around a 12" PowerBook for years, then needed a bigger screen with a 15" MBP (with drive). At some point I wanted something lighter again, so I used a MBA until the 13" MBP with touchbar came out in 2016.
I'm a researcher/teacher at a university. The 13" works for lecturing and in the office (hooked up to a 32" 4k monitor), but whenever I'm somewhere else (conference, working in meeting rooms with my team, etc.) I feel the urge to have some more screen estate, especially when a lot of applications are open. Xcode is usually good, I'm doing machine learning research mostly in Python, Matlab and R. Bigger models are run on a cluster for obvious reasons and I have a 16 Core + 1080Ti linux machine under the desk in my office. The screen size of the 13" is not a deal killer, just inconvenient from time to time.
I do my literature research mostly on a 9.7" iPad and occasionally use the 12.9" iPP for teaching (drawing graphs, showing some step-by-step calculations to students). I'm planning to pick up the next 10.5" iPP, if the bezel is smaller and home button is gone and feel like I could get away writing with it while travelling, while keeping the MBP in the bag. When I'm working with the MBP I usually have something to put it on (mostly desks/tables, sometimes on a plane/train), but no external display to attach it to (except for the office/home).
Would be a no-brainer if Apple would use Nvidia GPUs since it would support Cuda. The dGPU in the 15" would help a little but not as much as I'd like.
Out of curiosity, are you guys using a messenger bag or backpack to carry around your stuff? Using a messenger bag and when it's loaded (13" MBP, 9.7" iPad, power supplies, some cables, bottle of water and some accessories) my shoulder/back hurts a little after walking around for a while. Probably because the weight is pulling to one side, so I don't really want to add more weight with the 15". On the other hand, I've been thinking about going back to a backpack, which will distribute the weight a bit more evenly, so a 15" might not be that bad.
Decisions, decisions...