I personally love the Omega driver set, the catalyst control center is just too blocky and its feature set isnt too "featurefull".
the omega drivers use ATi Tray Tools as its control center, this can be downloaded and used with the normal ATi drivers but I also find that the Omega drivers are more stable and produce less artifacts with my overclocked gfx card, I dunno why but it just seems that it does. There really isnt an FPS diff between Omega and ATi drivers but I find the feature set of the ATi Tray Tools a HUGE improvment over the Catalyst Control Center. The ATI Tral Tools software has pretty much everything hardware(mainly graphics) related roolled into one.
You can monitor temps, overclock, adjust contrast/coloring, rotate your screen image, use hotkeys for things, show FPS in game (much like FRAPS), record a screenshot or audio (no video support yet), activate game profiles, tweak internal DirectX/OpenGL settings, enable refresh rate lock, etc... also, i found a really nice thing about it, i could hit alt+tab to minimize a game and enable/disable or change any feature like antialiasing or Anistropic filtering on-the-fly and go back into the game and it will be changed.
I'm relating the Omega drivers with ATi since one, i use an Ati card, and two the Omega driver set was originally designed on ATi drivers and later the Omega guy modified nVidia drivers too.
Though as for the cards themselves, it really depends on personal preference and the games you play since some are geared to play better on ati or nVidia. You cna see these games that say "Plays best on nVidia cards"(sometimes it will say "nVidia, the way its meant to be played") or some say "Plays best on ATi cards" A good example is that HL2 plays better on ATi cards. I cant think of an nVidia game off the top of my head but I know there are games that play better on nVidia, i think Doom 3 plays better on nVidia. Just do a little research