you should also think about tread. If most is mostly packed dirt (so it's almost like paved), then slick is good. I tend to throw on touring tires with a tread (conti touring plus 28s in my case) for wet weather an/or more loosely packed stuff. It makes some difference in how much the bike "floats" on the gravel before it sinks in, and that can make the difference between a fall and catching it.