I'm going to use it for work with game engines and Blender (modelling tool). Photoshop (not large files) and sometimes Premiere Pro.
This laptop won't be my main working machine, but I still want to be able to do some "serious work" even if I'm not at home. It seems that 13" is quite good for this too (rendering time: 13" = 35min, 15" = 23min)
8GB RAM is enough for me, but I want to get 16GB cause I'm buying this laptop for 3+ years
I love 13's compactness, but 15's screen size and power is also nice. So...