Whats the difference between an advertising sales agent and a advertising manager?

I'm choosing coarses to take in my 3rd year of high school...and I'm REALLY interested in advertisting...what's the differece between these two jobs? Which one is harder?

I generally have an A average in school. Which should I choose?

An advertising sales agent sells advertising to people. Let's say that the company is a newspaper. Then the sales agents are trying to sell advertising to all the businesses in the town. The manager supervises the sales agents.