I assume the answer has something to do with WMP being far more integrated into and designed for the OS it’s loading in that itunes. But I want to see the answer anyway.
I just tried this on my XP system.
Windows Media Player (whatever the current version is, I haven’t been paying attention) - 4.1 seconds to load.
iTunes (again, current version) - 5.1 seconds to load.
I would expect that iTunes is larger, since as you suggested, it loads QuickTime (which in turn contains a lot of “mac compatibility” APIs), but the time difference is not terribly significant in my case.
I will point out that I have no music libray to speak of in either application (I keep the music library on my Mac, and use the iTunes streaming feature to play it from the PC if I need it), so there might also be issues with loading the play list.
Actually, that’s pretty likely – the library isn’t on the main screen in WMP, and is on the main screen in iTunes. And WMP takes a fair amount of time to scan it when you switch to the library (or did, back when I used it as my media player).