They are inherently different.
Prostitution one party gets paid for sex, while the other is paying that party for sex.
Porn - The "stars" are paid as actors to shoot a scene where they have sex. They also pay the light guy, camera guy, director, etc etc. They are only paid because they know the movie is going to make money. If no one bought porn movies, then no one would get paid to make them. Also having scene the behind the scenes of a porn movie being shot, its not just two people having sex like the resulting movie makes it seem. They take direction, they stop, reposition, they do re-shoots sometimes. Its more like shooting an action scene for a movie, than a couple having sex.
Also prostitution isn't consider a safe profession. In porn they require regular testing, many studios require condomns, and the environment where they shoot is safe. No one is going to harm either party. Where as in prostiution, the work is done at night, usually late at night, the two parties most likely aren't acquainted, neither knows if the other is clean, and the safety of either party isn't guarenteed.
While porn might not be considered a "respectable professions", its not a visable profession. Most of the making of a movie is done behind closed doors where as in prostituion, the hooker has to pick up her trick, and thus is out for everyone to see. No one wants ladies of the night walking around on their streets.