Pre-ban/post-ban doesn't mean much anymore since the 94-04 ban expired.
For a while, pre-1994 rifles were selling at a premium since they had all the "evil" parts making them "assault weapons." For AR15s, the biggest thing was removing the bayonet lug and flash suppressor (or barrel threads for a flash suppressor). Unless you live in a restrictive state, it is legal to have these again. You can retrofit a "post-ban" rifle if you want, but most people don't bother.
There are many who would argue that the ban helped the AR15 community since removing the flash suppressor placed more of a premium on accuracy rather than military.