As a course developer and promoter, I can say that on the face of of it, promoting can be easier than developing one. The upside of development though, is that once it’s done, have to do is keep it updated and it can earn you money for years. That’s the case with my SEO copywriter training course, which I’ve been teaching (and earning from) since 2009.
It’s also important to research their payout structure. Not all payouts are the same and tier affiliate marketing often offer different payouts. Some might pay you on the first offer but pay you nothing on any upsells. Ideally, you should be paid for every transaction. In doing your research, you’ll find out which affiliate payout structures are the most profitable.