So far i have found Xpress Pro to be very stable. When I did my research I had decided on going Avid. My friend however went to FC Pro. After about 1 year, he went in had to edit on an avid for 3 days, after that he switched to Avid.
While there is no doubt that FCPro can get a job done,Avid has in my opinion a much better work flow, media management and extensive tools that appear to have been designed with editors in mind. The whole key to using this on XP Pro is doing all the tweaks to the OS. Videoguys.com has a pretty good list to start with.
hope this helps.
R