What good is an operating system without software that runs on it? No good at all. So it's worthwhile looking at how compatible existing software is with each operating system.
When it comes to compatibility with existing third-party applications for the Mac, Snow Leopard has some problems. As I've written in Snow Leopard: Which apps, utilities have been left behind?, I found a number of compatibility problems between Snow Leopard and software that I use on a daily basis. The excellent Xmarks bookmark synchronizer won't work on Safari in Snow Leopard, for example, and neither will the free office productivity suite NeoOffice. In addition, Adobe Systems has said that its Creative Suite 3, which includes Photoshop, may not run on Snow Leopard, although Creative Suite 4 should have no problems.
I've found several other utilities that won't work either, such as the very good Windows Sync synchronization tool from Windows. In some instances, betas of the utilities or applications already exist that fix the problem. In others, work is being done or planned, but not yet completed. Most likely, most if not all popular applications will eventually be compatible with Snow Leopard. But that's not the case today.
As for Windows 7, Microsoft seems to have learned the lessons of Windows Vista. Applications written for Windows Vista will work with Windows 7 -- I haven't found a single issue where that isn't the case. There's also a Windows XP mode that allows applications written for XP to run on Windows 7 and look as if they were running on it natively. True, it's a kludge, but at least it works. (Note that XP mode is available only for certain hardware and in certain versions of Windows 7.)
The Winner: Windows 7. That's for now, though. In relatively short order, Snow Leopard may have its compatibility problems worked out.
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 32-bit version requires 16GB of hard disk space, while the 64-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.