I run 700x35 or 700x38 on my main rides, which see mostly paved back roads with a good share of rail trails thrown in, and some gravel and hard pack, but not too much. They seem to handle the highly variable surfaces well, and I rarely experience flats. The ride is pretty comfortable, too. YMMV. I am a recreational rider, not trying to burn speed or watch every ounce of weight on the bike.