The extent to which you enjoy economics will highly depend on your teacher and natural skill in the subject.
I had an awesome teacher, and my class learnt virtually the entire course (including HSC) doing multiple choice and short answer questions without any boring textbook work. So it was great. Be prepared for a larger workload in modern history. Economics is concept-based and everything links together, so if you grasp it easily there's seriously little work.
You should know whether you would like modern history or economics by your general interests and curiosities.