I am currently running Michelin tranworld cities and find them to be fast on the street and still give plenty of traction for non-technical off road riding...
http://www.bikeforums.net/showthread.php?t=357127 . You could go with something more knobby but I would not if most of your riding is on the street.
I am running 32's but if I were you I would probably go with 28's for street performance.
Based on reviews, it seems you really need to run a tire liner with these. I am doing so.