Leopard Upgrade vs. Windows Upgrades
While I had some trouble with my Leopoard upgrade, I never had a windows upgrade work nearly as well (though I have never touched Vista nor even know what it looks like). I also never had luck moving programs and data from one PC to another during a windows install.
The Leopard archive and install worked very well, and the transfer of my files and programs from my mac mini to my imac when I upgraded was near flawless (A few pesky PowerPC apps did refuse to run on my intel imac).