If you really want to compare the difficulty based solely on how complicated is the mathematic; my answer is engineering. The difficulty of add math in engineering course easily kill those best students. While on the other hand, ACCA is pretty much tolerable even for students from the lower grade classes. Based on my observation, most of the students in the first class during my high school is doing engineering. Those in the third class and below went for the accounting. It telling the difficulty there.

Anyway, like many has said. Both courses are in their own world. They are both useful to the society. Of course if you final goal is to make money, you may as well join gangster and do some illegal business. But my true heart will say, your life shouldn't only revolves around the money, there is always better thing for you to sacrifice than making money. Only when only you found the 'better thing', you will appreciate your life better.