64-bit XP has a reputation for being more difficult to get device drivers for. 64-bit computing didn't really become popular until Windows 7 because the price of 4GB+ of memory became reasonable around the time it came out. As a result it's a lot easier to find 64-bit device drivers for Windows 7 than for 64-bit Windows XP.
Windows 7 also contains built-in drivers for most newer hardware, including SATA and the newer chipsets. That means it's less hassle to install on modern computers.
For those reasons, I suspect most people would prefer 64-bit Windows 7 over 64-bit Windows XP.