How much portability do you need?
I once got the 15" and had it for a week but realised for me it was too big to carry around a lot and move about. But too small for a desktop replacement. So I got an iMac.
Now I'm sellling the iMac because I will need portability for work this year. Plus I want to browse the net on the sofa!
I'm pretty level headed and make good decisions so I'd say if u want future proof power get 15", but if u want to move around with it a lot then 13" is for you. I'm buying a 13" soon, put it that way, and I use it for similar things to what you said.
The problem with the 13'' is that I doubt it'll be powerful enough to be my only computer. If it wasn't for that, I'd definetly get it (since I'll get an external display screen anyway).
Side note: My current laptop is 2.8 Kg, so the MBP 15'' is around 300g lighter, and the MBP 13'' 800g.