I've found merino to have a "drier" warmth to it than synthetics. Even when you are kicking off excess moisture via evaporation it stills feels warm(er) to the skin versus the evaporative cooling effect with the synthetics.
Though not as soft - and can get that wet dog smell when wet (even when brand new), I've found the natural fiber better at keeping the inevitable plastic stink at bay.
Merino might not be as durable, but then again it isn't made out of recycled water bottles.
