Obviously sex with attraction on both sides is far superior than sex with a hooker. The thrill of the chase, finding someone new and then banging her is, far and away, more exciting than sex with a hooker.
The first 6 months or so of sex with a new girl is magic - the sex gets better and better. For this reason, a real relationship is superior to using hookers on a routine basis.
All of the above are preferable. With that said, sex with a hooker can be awesome as well if you get stuck in a pinch - girlfriend and wife are on the rag - or both at the same time. You haven't had sex in a week or two for whatever reason or maybe you're out of town.
Like someone else stated, it is like a service - like getting a haircut.
Just make sure you don't go bottom-dwelling. You get what you pay for. If you pay for a beautiful girl who gives GFE, you'll feel amazing coming out. If on the other hand, you pay $100 for 15 minutes with a fat ugly girl who has a bad attitude, you'll make yourself feel like shit.