I've ridden a lot on unpaved trails and roads, and I prefer at least 32s and 35s are better. Some tread is also important if there's any chance for mud or wet spots. I rode the entire GAP-C&O Canal trails on 32mm Panaracer Paselas one summer, and on Clement LAS 35s the following summer. The Clements were much better, particularly in mud and loose gravel.