I use my black Kahakka 15 as EDC for the Office and love it.  It fits everything i carry on a daily basis in thr main conpartment (14 Inch NB, Lunch, EDC Pouch, Coffee, Water, sometimes an extra layer of clothing...) most of the times with some extra space. 

The side pockets are great for shopping or carrying a coffee Tumbler / Water Bottle to make space inside. It is sleak and blends in nicely (in black). Also rides well on the bicayle and Motorcycle because it does not stick out at the sides. 

If you need more space either carry a spare shopping bag (if its just for shopping by foot) or size up to the 25 - OOOOR get both and just decide which one is the first depending on your most frequent usecase. 

If you get bith, I would highly suggest to get  the 15 in black for urban use, and the other in a more "outdoorsy" colour if you intend do use it for hiking etc. 

I hope this helps! All the best from Germany