With Snow Leopard finally been given a release date and a price, the comparisons with Windows 7 are starting to pop up all over the place. Everyone is asking “Which one is the better upgrade i.e. which provide more value for your money?”
Snow Leopard is truly a release where the under-the-hood stuff is more important than user-visible changes. OpenCL, Grand Central Dispatch, removal of PowerPC code, fully 64bit, they all aid in making Snow Leopard faster and leaner. There are also some interface tweaks, but they are small (but useful!), and certainly don't draw that much attention.
Windows 7 is a completely different story. Microsoft has made the operating system perform better than Windows Vista (and every report confirms that per osnews research team), with some even claiming performance on par with Windows XP, especially on more recent hardware. Snow Leopard could give a more substantial improvement in this area, but only very recent and powerful 64bit machines will see these benefit. Windows 7's (possibly) more modest performance gains over Vista will benefit machines that are much older and/or much less powerful. For instance, both that old Pentium 4 box as well as low-spec Acer Aspire One perform better with Windows 7 than with Vista. Similarly aged and/or spec-ed Macs can't even run Snow Leopard at all!
However, where the difference really becomes obvious is the interface changes Windows 7 introduces, and all the pulling-together of frameworks and features introduced with Windows Vista, and exposing them to users in much more useful ways than Vista did. The best example of this is Homegroup, which pulls together various technologies and features introduced with Windows Vista, and presents them in a way that makes managing your network and shared files completely painless. Another example is that various driver upgrades no longer require a restart, such as graphics drivers.
The interface of Windows itself has also been massively cleaned up compared to Vista. It really takes some intensive usage to reveal just how much of the interface has been cleaned up, made more consistent, and overall prettified without losing functionality. An example of this is Windows 7's Explorer file manager, which is such joy to use now, in comparison to Vista which was a busy and clumsy mess. Of course, there are still trouble spots, such as the overly crowded Control Panel.
Snow Leopard simply doesn't bring these kinds of massive interface improvements. It can thus be concluded, Windows 7 seems to be more substantial.