I believe the main distinction is this: In porn everybody involved is getting paid for their efforts. There are business contracts and such. It's considered "entertainment" and the people involved are "performers" or "actors" instead of sex-workers.
In prostitution one person is paying, one is "performing." An imbalance of power.
An interesting question would be: If a man pays a prostitute (in an area where it's against the law), films it (with her knowledge and permission), then sells the tape for profit, is that considered more illegal or legal? Technically: they both have given consent, and are both getting paid for it.
Native XWA.Netter (Nutter?)