I have never really thought about it before, and this is a personal view:
XT600 - Dual sport / on & off-Road;
TT600 - Purpose built off-road machine (i.e. no cush rubbers used in the final drive, etc...);
Tenere - Enduro version of the XT600 (based upon Paris Dakar versions with larger tank capacity & longer fork travel, etc....).
n.b. All can be modified, and road going TT600 can be a great machine. A Tenere can be a good 'green lane' machine, and an XT600 can be Tenere'ized, etc....
Hope this helps? Perhaps others can add?
If you find a good machine, there is alot of fun modifying it to your needs/wishes, as they are all the same basic core design. Again this is a personal view.