This is a classic question and there is a lot of history to it - rooted in the early days of adult content production (before internet).
In short, the main difference between the two is that adult content production is oriented to the production of erotic material - a tangible end-result of the interaction, whereas prostitution does not.
This is one of the main tests to separate the two.
It's moot in Canada, in that prostitution is legal here as well (you just can't discuss or make deals about it in public or private - that is solicitation which is against the law).
-Dino