en.wikipedia.org/wiki/Friction

I believe this is a question of physics and depends wether you are referring to object that is begining to move (static) or a body in motion (kinetic). The formula to answer your question is:

assumin a flat surface: force= Mu X wieght. where Mu is the coefficient of friction. The question to you is which Mu do we need, Mus= coefficient of static friction of sand and water OR Muk= coefficient of kinetic friction of sand and water

In other words is the car moving and you are hitting the brake OR are you beginning to accelerate?

I miss physics!

My gut feeling is given equal amounts of "contamination", the wet track gives less traction. Just thinking out loud; sand might create a unique challenge as wind shifts could alter dusty areas and amount of dust on the track at a given time. Opportunities to practice for those conditions are also more limited.

It was interesting that during todays Bahrain GP there were comments that at least a couple of teams engineers were seeing little difference between the hard and soft tires for at least a couple of the teams.