Which is more difficult 'Developing' or 'Marketing'?

I think it can depend on the specifics of the product, but for most products it's probably true that the hardest part is marketing - finding the right channels (and understanding when you need to work harder and when it's just not the right platform for you), making people believe in the product and pay for it. Usually, making at least an MVP isn't that hard, but proving that it's worthy of upgrades is another thing.

Depends on your skill set? I think that proving though that any MVP is worth putting money into is hard!

Marketing