From an update point of view, Apple rolls out new OS builds every month or so it seems like. Windows has a bit more stable upgrade path. However, Vista is almost three years old now, and is due for replacement in about another year (with Windows 7).
Vista, with SP1, is actually pretty good. Its still not quite as fast as XP, but has a lot of nice features to it that really do improve the user experience. Also, Vista is pretty easy to use. The "new" things are all fairly intuitive.
From everything I have read on the subject, Windows 7 is just going to be a streamlined, reskinned version of Vista, with a bit more polish on the rough spots.
The next major change in home computing that will complicate things is the 32bit verses 64bit operating systems. Windows 7 will be available in both, but many other things might not be. If you are doing hardcore applications, this is something to seriously think about. 64bit has the ability to address more RAM and run faster. 32bit is limited to 3GB of RAM, 64bit can go MUCH higher in the RAM department. New motherboards have 6 slots these days!
I love Mac, but they age very quickly. If you are going to keep the box for a long time (my goodness man, 9 years?), I would recommend Windows because it will age better and be upgradeable for a longer period of time.