Well, win7 can do an in place upgrade in vista, but XP doesn't so none of your settings are migrated. With an in place upgrade most settings and programs will be intact instead of having to reinstall all your programs. With an XP upgrade you'd be forced to reinstall programs anyhow.
So, I guess an upgrade disk is worth it if you have vista and don't want to do a clean install. Also, upgrade disks are less expensive.