Interesting topic Rexroth.
As for designating us johns, there is also the term "trick", although that term might primarily refer to the act, from the SP's perspective, rather than its object. On a scale ranging from the derogatory to the elevating, I would place:
trick-john-monger-hobbyist
Now on the supply side, we could place:
whore-call girl-escort-sex worker-service provider-courtesan-travelling companion
Each of these terms, while possibly referring to the same person, carries its own agenda. Here are a few that spring to mind:
Whore: term used by men who have never met SPs and by wives whose husbands see SPs.
Escort: term that originally seeked to give the impression that sex was not the primary goal of the encounter, as when a man ask his female co-worker to escort him to a business luncheon.
Sex-worker: neutral-sounding term used by government do-gooders and Stella-like advocacy groups. The agenda behind the term is to bring SPs into the mainstream.
Service-provider:term probably invented by the techno-geeks that inhabit these boards who can't distinguish a tIT from a IT vendor.
Courtesan: an SP that has the ability to titillate your brain before sucking your dick.
Travelling companion: an overpriced SP
There you are
Gotta run off to lunch (with my escort) but I could think of several more.