Which of the two sizes (notebook) would you recommend?
For university?
15.6 in.
If you only want to use the device at university and still have a monitor or PC at home, 13/14 inches are completely sufficient in my opinion and are more handy.
However, if you don't have a monitor, then I find it very stressful to work longer with a small display. Especially if you write a lot and do research on the side and even want to look at 2 windows at the same time. Then I would put the advantage of a large display (15 inches) over the disadvantage of handiness.
So I had a small 10-inch notebook just for taking notes at university and a 17-inch laptop at home for real work… If you only want 1 laptop, I definitely recommend the larger version. Working is much more pleasant with a large monitor.