As somebody else said, tread is pretty much irrelevant on road bike tires--the advantage of tread voids (the open spaces) on cars is that the water can escape through the channels, it doesn't "wedge" under the tire and cause hydroplaning. Bike tires are so skinny that's never a problem.
A softer rubber compound may give you better traction in the rain. Personally I've never been able to feel a difference, but I'm just riding around, not trying to corner as fast as I can.