I would try both. In PRman there is a parameter in the Environmental Light called MaxVariation. Let's say we want ambient occlusion in our scene. A MaxVariation of zero will calculate all the hemispheric sampling of required to created nice smooth ambient occlusion, which of course takes oodles of time to calulate.

As you increase the value of MaxVaration, hemispheric sampling decreases and the renderer interpolates values between the samples based on shading rate. So values of >=10 for draft renders, a value of 1 for most final renders, occasionally lowering MaxVaration towards 0 in the most demanding scenes (for example when many, many objects are very close together).

Many of PRman's coolest effects (including raytracing) can be cached and pre-processed, written into "3d textures" for use in subsequent renders, which I believe is what Firefly does in one step. Like shading rate, a lower value may equal higher accuracy. Try it the first time with a value of one and again with a value 10. Look at the resulting renders and I'm sure you'll be able to see the difference.