pick a set-up that's in the middle, so yeah it'll be a bit of a comprise, but if you have a fully dry set-up and then it rains, you might really struggle for grip.
If you go for a balanced set-up then you might loose a bit of time in the dry compared to the fully dry set-up, but then have reasonable grip levels in the wet. You'll just have to balance it out.
Plus if it looks more like one condition than the other, ie 80% rain chance, maybe go a bit more towards the wet set-up cause more of the race could be run in the wet.