I really liked doing diagon alley first just because it makes sense with the Harry Potter story. Go to London, then to diagonal alley, then take the train to hogwarts. Yes, I'm nerding out there, but that was important to me
As for food, I much preferred leaky cauldron to the three broomsticks. Ate at both and found leaky cauldron to have better quality food, but I think it might have a smaller menu.