Winamp3 in my mind sounds inferior (sound quality) to WMP 9. But, when your playing games, running Winamp at under 1% cpu usage compared to like 20% to 30% cpu usage, makes quite a big difference.
To some of my friends, this skin
(Click Here To See It) changed the mind of plenty of my friends. I think the choice has partly to do with the presentation or in this case the skin itself. WMP 9 is now also implementing some features where it is starting to recognize which files you downloaded illegaly and which ones you bought.
Winamp gives more plugin capability, skins, add-ons, etc. Check out their website at
www.winamp.com