From your experience, what are the advantages of paying for a MOOC compared to taking the free version?
This is actually a very difficult question to answer as it depends on so many things, the main one being the REASON you're taking the MOOC in the first place. If it's just for personal curiosity or interest, then there are no advantages to paying as you have access to the course lectures in the free tracks.
If you're studying for career advancement, then, again, it can also depend on the course, who the provider is, how much depth it goes into etc etc I don't think adding MOOC certificates (which would mean paying) on your cv works in your favour UNLESS you can demonstrate work you have done as a result of taking the course. For this reason, I would advise looking for courses that have a final project that you can then share to demonstrate your skill in real terms. Usually, accessing final projects is a paid feature and can be worth the cost if you're doing the course for professional reasons.
Another well-known advantage of paying for a MOOC is that you're more likely to actually finish it!
I hope that was helpful...