I have lived in Knoxville most of my life but have tons of friends in Chattanooga and spend a fair amount of time there. Ill try to be objective. For an in-town mountain biking scene Knoxville blows Chattanooga away. We have more trails within 20 min. of town with regular group rides from mild to wild several night of the week. The MTB community in Knoxville is very tight and within no time newcombers that want to can make tons of friends and step into the scene easily. Trails near town vary from fire road fun to twisty singletrack to the best DH runs in this part of the country. Within a 2 hour drive of Knoxville lies the north Georgia trails, Tanasi, Tsali, Pisgah, The Big South Fork, Damascus and more. Several of us in town travel to ride on the weekends as much as possible. There are a quite a few racers in town if that’s your bag. The web site mentioned above is a great tool for the Knoxville community to communicate. Though they don’t allow mountain Biking the national park outside of Knoxville is a big plus in my book for other recreation.
Chattanooga is a good bit different. The MTB scene is not as centralized and the MTB community not as tightly nit. I think its probably because they don’t have as much riding right in town. Within an hour drive they have tone of great stuff but its tough to drive that far after work every day. The road scene in Chattanooga is hot though. If group road riding or racing is one of your interest Chattanooga is going to beat out Knoxville for opportunities in that direction. There is allot of riding within 2 hours of Chattanooga in North Georgia, North Alabama, Tanasi, Franklin State Forest etc. but you’re a long way from Pisgah.
As a long time cyclist and White Water enthusiast I have always appreciated Knoxville’s central location to most everything the southern Appalachians have to offer.