it is easy: high PT samples = bad AA/MB/DOF, good lighting, and vice versa. Then in lighting, high lights multiplier = good direct lighting and bad indirect (GI), and vice versa
It could be good idea to create some sort of flow-chart where people would navigate to what sort of scene they have: ie. exterier/interier/natural light/artificial light/many-few lights/dof-no dof/etc..and in the end would be suggested bracket of values of these paramaters.
I think there is currently lot of confusion in how to trade between PT samples/Light samples, and what would be some sort of "universal" settings akin to traditional unbiased renderers, if one wanted "one-button" solution (or is the default closest to this?)
The flexibility of Corona is amazing, in that, you tweak as little or as much as you want, but currently the guidelines for tweaking are blurry in the documentation.