I haven't given Windows 7 or Snow Leopard comprehensive run-throughs that test how well they work with different peripherals. As a result, there's no way for me to compare their compatibility with peripherals at this point.
But I can generally gauge their compatibility with the hardware for which their operating systems were designed. So how compatible is Windows 7 with Windows-based PCs and how compatible is Snow Leopard with Macs?
Snow Leopard requires Intel-based hardware; in other words, it won't run on PowerPC G4 or G5-based systems. This means if you've got an Apple machine built before 2006 (when Intel processors were introduced to Macs), you're out of luck. The various Mac lines were switched to Intel processors at different points throughout 2006, so if your machine was built in 2006 it may or may not be compatible. You can check About This Mac under the Apple menu to see what kind of processor you have.
In general, though, if you've bought a new machine in the last three years, you're probably in good shape. Snow Leopard requires 5GB of free disk space and 1GB of RAM.
With Windows 7, things get a bit more complicated. It is designed to run on any machine that runs Windows Vista, which was released in January 2007. But it will also run on many machines originally designed for Windows XP -- in fact, I run it on a Dell Inspiron E1505 that I bought before Vista's release in January 2007, and that was originally an XP machine.
To be more specific: Windows 7 requires 1GB of RAM for the 32-bit version and 2GB for the 64-bit version. The 16-bit version requires 16GB of hard disk space, while the 32-bit version needs 20GB. To run the Aero interface, a graphics card must support DirectX 9 graphics and have 128MB of graphics memory. These are generally modest specifications, so many PCs designed for XP can handle Windows 7. That means that Windows 7 will run on older hardware than Snow Leopard (although if your machine dates from 2001 or 2002, you may need to check its specs carefully).
The Winner: Windows 7. Windows 7 will work with a wider variety of hardware for which Windows was designed than Snow Leopard will work with Macs.
Ease of use and elegance
Windows has come a long way since its humble -- and let's face it, just plain ugly -- beginnings. With each iteration of the operating system, it gets a little slicker, a little smoother, a little easier to use. Windows 7 continues this tradition, particularly with the new taskbar.
All that being said, Snow Leopard, like earlier versions of Mac OS X, is just plain beautiful. The word "seamless" is overused when describing an elegant, simple-to-use product, but in the case of Snow Leopard it's absolutely true. It's as intuitive and as aesthetically pleasing an operating system as you can find.
In addition, because Apple controls the hardware as well as the software, the integration between machine and software is unparalleled. Windows users have become used to strange behavior and odd error messages that appear from time to time -- it's the background radiation of Windows. To a certain extent, there's no way around it. Windows has to work with countless different combinations of CPUs, RAM, hard disks, video cards and other main system hardware. Because of that, these kinds of problems are almost inevitable with Windows-based hardware. They don't happen on Macs.
The Winner: Snow Leopard. No one beats Apple when it comes to design. As with previous versions of Mac OS X, Snow Leopard is flat-out beautiful. Windows may be improving, but it still has a way to go if it wants to catch the Mac.