Tread actually does make a difference in the wet. Namely, it reduces traction by reducing contact area. Slicks with round profile are the way to go if you're staying on pavement. I've been very happy with Conti GP 4 Seasons and Schwalbe Marathon Supreme for wet use. Both are pricy but I don't think riding in the rain is a time to go cheap on tires. (Actually, I don't think any time is a good time to go cheap on tires, but in the rain I can use safety a an excuse.)