Windows 7 has a lower internal version due to, per Microsoft, a lot of bad installers out there which do simple, stupid version checks which would break if the internal version was set to 7. Same reason they put the memory limit on Windows XP as of SP1: shitty programmers.
Windows 7 is leaps over Vista in just about every aspect, and I would not call it a Vista service pack anymore than 98 was a service pack for 95, or 2000 for NT 4. I used the betas and RCs of both Vista and 7 and, even in beta, 7 was much, much better. The only two things that keep me from moving to 7:
1) No in-place upgrade from XP. IMNSHO this is a big FU to everyone who refused to upgrade to Vista and there is no legitimate technical reason for the limitation.
2) I absolutely despise the Vista interface and that I must work my way through the equivalent of a phone tree to get to functionality I want, especially knowing it WILL change with the next Windows release.
XP x64 works very well for me, so I will stick with it until I am in a position to reload my machines or I absolutely have to move up. Renewing my MAPS subscription may help a little, too, since I have let it lapse and do not have any of the newer software