Both of the UI are slow, however OSX is slower. OSX has built in apache web server, is built on unix (bsd, nextstep) core. XP is easy to use, more programs available. OSX is even easier to use less programs available but all of the big names are there Office v.X, IE, illustrator photoshop(soon) etc...
Both of them are better than the OS they replaces (Dos(9X) and OS9). Feature wise i think OSX is ahead.
Basically If you want compatability go with windows, if you want style/ease of use go with OSX.