Which capacity measurement is the correct way to measure?
Why is it less in mAh but more in Wh? I get that it's because of voltage but that's not the question.


It might not be the question, but it is the answer.


  If I were to somehow use my laptop's 3100 mAh (42Wh) battery on my phone, would it last longer or shorter?


Wh is a measure of the amount of energy stored in the battery. With an efficient voltage converter the battery with the higher energy would power your phone for longer.