Obviously they all refer to the same thing, some are more charged than others. Some ladies seem to feel empowered by fully embracing the "Madonna-Whore" dichotomy and others dislike the more negatively connotated words and find them distasteful.
I agree with Caroline though, ultimately it's all about the intent and the context in which the words are used. Almost any of them can be used to either denigrate or merely describe depending on the context.
I tend to be more formal and professional in my communication style so I stick to "Provider" as that's the universally agreed upon PC term on the boards. BCD though in a more PSE type session I like to talk dirty and do use the word "whore" from time to time. It's got little to do with her profession though and more to do with what she's doing for me at that moment and the fact that the word isn't acceptable in polite society as a whole so it's sexy to me. I use it with freaky civie girls too. Most ladies understand this as well and don't take any offense to it in the heat of the moment, to the contrary actually.
I'm unconcerned with the terms used to describe me as a client. Frankly none of those words have much negative connotation in my mind. Even if they did I could care less, it's not going to hurt my feelings any.