To me, FM was much harder than physics for the following reasons. Physics is, conceptually, harder; the questions require more thought and, for lack of a better word, intelligence. However, in my physics exam last year I pretty much lost around 10-11 marks and still got a 100/100 ums. On further maths, I made a stupid fkn mistake in my mechanics and lost 4 marks to get 88/100 ums in that module. So physics was much easier in that sense, it gives you more space to err.
Conceptually, though, physics requires original thought most of the time. There are standard questions that guarantees you a high B or a low A, but to get the A* you need to be able think well.