I have finished the sap hr course few months ago and preparing my self to sit the exam
I have asked the same question a lot of times before I decided to do this course.
Answer: it all depends in your country of residence in europe:
payroll is no 1(very complicated) time management follows (less complicated than payroll)
personell administration and org management are basic modules which doesn’t need alot of knowledge
benefits is used in few countries like usa and canada
I would suggest you to take the payroll if not time management
I hope this wil be usefull to