Both work well; slight advantage to Ubuntu IMHO.
I am damn close to wiping Windows off my parents' laptop and replacing it with Ubuntu Linux, just so they'll stop calling me and complaining about how slow both the machine and "the Internet" are.
Can you give any possible technical explanation for how the operating system on a given PC could affect the speed of data from the Internet to the network interface?
I'm a little curious why you would benchmark an eight year old version of Windows and present it without comment, as relevant.