I run a 26x1.5" slick in the front for comfort and maximum rubber patch on the road. The rear is a 26"x1.8" with deep tread and a profile similar to a slick (negative tread?). It's comfortable and still has bite when I hit loose dirt or very wet grass. I cut across a local park to avoid a bad intersection.

This combo works for me but it really depends on the path you use. If I was always on nice clean asphalt I might be happier with a pair of 700X32 on a road bike.