You are correct that Vista isnt as supported. But it is stable and as bug free as XP is right now. Actually there are still more hotfixes being released for XP than Vista every month thru Windows Update.
The only downside of Vista is that since it is coded totally different that many software and game developers are slow to get applications and games converted to Vista.
Till they get things Vista compatable XP will still lead in the support group. But overall Vista has been more stable and has less bugs to date than XP has. Even since Janurary when it was released.