i'd say humanities are much easier, from my own experience.
i did all three sciences and english lit for a level.
all the year, english has been my doss subject, and yet, when i talk to the other people in my eng group who all do humanities, they say english is their hardest subject.
it depends on the person; some people are more scientifically inclined, some not.
but in general, it is much more difficult to blag a science/maths exam. there's far less room for opinion than in humanities. furthermore, to reduce the subject to "memorizing formulae and using them" is incredibly ignorant!