I can touch on this from a massage angle. Im sure some of the same theories apply for SP's....
As a provider of any kind, you have pros and cons to both...Indy's can set their own schedule, their own prices, and possibly not have to travel far (depending on incall/outcall). With a spa or agency you have power in #s. The advertising dollar that an agency or spa have is bigger than any Indy marketing budget. Girls have saftey in #'s, and a set location. Once booked for their shifts, they dont really have to juggle or think it through. Reception books your appointments for you. Also, Indy's have no fillers for No-Show appointments, and at a spa or agency incall, walkins can replace no-shows, or simply add #s to your day. In addition, your supplies and facilities are stocked and furnished for you in a spa/incall. It costs less to work for someone, yet Indy you pay no house fee. (ie: doorfee, agency cut, etc)
Its truely up to the girl and what works for her. Many go Indy and realize how hard it can be to maintain and run your own business. When you are with a client, no one can answer the phone/emails/inquiries. Perhaps they are losing business by simply being with a customer. Yet others have trouble conforming to rules within a location/under a manager. Spas dont tolerate lateness, repeated sick leave, etc.
Either way they work it, its up to the girl to get clients repeating their business with her. The rest is simply preferance and technicalities.
My 2 cents boys
Emily.