Generally I think smooth track = harder setup and bumpy = softer.

I tried a hard setup on a smooth but loose track and got a very consistent feel compared to a soft setup. Both setups felt loose but with the soft setup the car could suddenly dig-in in corners and just swing around. The hard setup took care of that and made the car consistent.
Once the track got rutted I should have switched back to the soft setup as I couldn't put the power down at all.