XP was originally more unstable but then Windows 7 hasn't yet had as much chance for instability to be updated in.
On cost, performance, control, ease of use, and stability, Ubuntu beats them both hands down. The only real problem for people moving to Ubuntu is the ubiquity of Windows. That means users have to learn a fair bit about Ubuntu. But then, they end up doing that with Windows anyway, turning off all those stupid defaults, hunting down where Windows settings are hidden, and getting around all those intrusive warnings and silly messages.
It's certainly worth the investment of time in Ubuntu to get away from MS. And you never buy software again.