The Omega drivers _are_ the ATI drivers, with various configurable settings
tweaked to provide better or more stable performance in games and some
other applications. The Omega drivers also have support for the Mobility
chips turned on, which the drivers on the ATI site do not.
Omega:
Optimizations for image quality + performance, included soft-mods for
some older ATI cards, ATI Tray tools (great app) built in, cleaner
install/uninstall than ATI drivers. Limited blessing from ATI catalyst
crew (they seemed to indicate they were in direct contact with Omega
guy in some forums a while back), minor bugfixes over ATI in some
versions/card combinations.
ATI:
Available before Omega versions, that's about it .