Unusual choice: Windows XP (x64) vs. Windows Server 2008 (x64)

"Windows Server 2008 have same base as Vista.

But if you have only this options, I recommend Server 2008.

Because XP (x64) is very old and MS support has expired and haven't new updates.

UPDATE: You mentioned ""web development"" and for this is good Server, because have hyper-v for virtualization."

"Windows XP x64 is built on top of an OS which is 8 years old; true that the x64 release was quite a way after.
Here are the system requirements for Windows XP x64 (via: http://technet.microsoft.com/en-us/library/bb457053.aspx)

Processor
733 MHz Intel Itanium
800 MHz Intel Itanium
Memory
1 GB RAM
1 GB RAM

Conversely Windows Server 2008 is built on top of the same kernel as Vista and has things such as UAC (love it or hate it UAC is a step in the right direction) and a better CPU schedular. Win 2k8 was build in the era of multi-core processing, unlike XP.
For the record here are the Win 2k8 system requirements (via: http://www.microsoft.com/windowsserver2008/en/us/system-requirements.aspx):

Processor
Minimum: 1 GHz (x86 processor) or 1.4 GHz (x64 processor)
Memory
Minimum: 512 MB RAM

My vote - Windown Server 2008."