I do both of these things...I like the safety and familiarity of a return trip, particularly the ability to have it be last minute without the need of a screening since I have a crazy schedule sometimes...but it can get stale just like anything (I think for both parties involved). So sometimes I mix in new providers so that even on my return trips with ATF's it feels new...
I also just have moods. There are some providers that I'll see anytime, any place...there are others I have to be in a mood for that kind of a session (and effort), depending on activities provided or the type of visual stimulation desired. Sometimes I want CFS, sometimes just a BJ, sometimes I want GFE/kissing, sometimes I want PSE, sometimes I want a dirty talker, sometimes I just want someone to hang out with. Sometimes I want to experiment with a new activity, sometimes I just want a release. Different providers provide different experiences, and whether I return frequently usually revolves around whether I'm in that mood frequently and not always whether I liked that particular provider.
Obviously, some providers will adapt their session to my mood, and those are usually my ATF's.