Which is better...

Well depending on your needs... For me I recommend Windows Vista Ultimate with SP1 if you have very good PC, because it's Windows, there are lots and growing of available applications for Windows Vista whether it's freeware or not; especially PC games, if you are a hardcore gamers well you'll probably love Windows Vista which supported all latest games in the market which exclusively has DirectX10+. It's also has a new redefined interface which is more easier and simple to use than XP did, it also has new features and applications like Windows Photo Gallery to organize you're pictures and videos from you Digital Camera or from DV Camera, Windows Movie Maker that is now Hi-Def, Windows Media Player which easily play media files, organize them, buy them and easily sync them to your portable media players, Instant Search which easily search files in fastest way because of indexing, Voice Recognition to operate your OS with your voice. There are lots of features and apps in vista. You will have a best looking OS in the world. Just to be sure that you reach or above it's system recommendations to faster performance and it has Service Pack 1 to avoid most common bugs and faster performace... Hope this helps! Cheers!