The German magazine c't has posted an article on what it considers a realistic benchmark rather than the ones used by ZDNet and Mindcraft.
Their test system was a Siemens Primergy 870 web server equipped with four Pentium II Xeon CPUs (450 MHz), 2 GB of RAM, and two network cards. RAID 5, rather than the RAID 0, was used for their disk array.
Conclusions.... For static pages, Linux performed comparitively worse when more than one network adapter was used and when most of the SMP load was from the kernel. However, when dynamic pages were delivered through CGI scripts, Linux and Apache seemed to have the upperhand.
Check out the complete article for a more detailed assessment of the pros and cons of each setup along with some tuning tips for each platform.
Also, check out Information Week's article by a couple of consultants and their experience with both platforms.