Going to be honest, never touched Vista unless I had to and I haven't yet touched Windows 7. I want to be able to do anything I want to with my OS, so WinXP was the last M$ OS I'll buy. With WindowsXP, multi-monitor support on multiple graphics cards works simply and reliably. In Linux, it works 'ok', but sometimes to get a good multi-monitor setup, it involves writing scrips to use 'xrandr' during boot up.
That doesn't keep me from using Linux, far from it, but Windows does do multi-monitor support better. As stated before, I only have Windows anecdotal evidence up to WinXP.