Historically, the 64-bit edition of Windows XP hasn't been a runaway hit. It's not really surprising in many ways: the x64 version was a rather specialist product built off the Windows Server 2003 codebase, which traded a loss of support for a number of features (including .NET Framework and DirectX in early releases) in return for the ability to address vast amounts of memory from a single process. Since the AMD64 and Intel EMT64 ranges of processors happen to run 32-bit code extremely quickly, it's not surprising that even though probably the majority of machines sold these days are 64-bit capable, they are running the standard 32-bit version of the Windows XP operating system, with the 64-bit edition used mostly for workstation-class tasks such as engineering and scientific applications.
In Windows Vista, the choice becomes far more pertinent for most people. Gone are the limitations of the 64-bit edition in terms of what software comes shipped with the operating system: a "Longhorn Basic" requirement for any feature to have been accepted into the operating system was to be supported on both 32-bit and 64-bit platforms. But the 64-bit version also takes advantage of the hardware to offer some attractive security features that are exclusive to that platform. On the downside, you'll still see weaker downlevel compatibility on the 64-bit platform: for example, we don't support 16-bit code from the Windows 3.x days (which is a little more of an issue than many people realize due to the inclusion of 16-bit code in the installers for some decidedly 32-bit applications).
The choice is an interesting one, and by far the best article I've seen explaining the differences between the two platforms in detail is from Paul Thurrott's Windows Supersite. It succinctly defines the pitfalls and strengths of each choice. It's also worth checking out the Windows Vista Product Guide for more details. I'm considering holding out for one of the new Merom-based notebooks that will contain dual-core, hyperthreaded 64-bit processors: so much power in one portable machine that it makes me drool!