Windows ve Linux
If you want to run the most powerful Windows based server then you really need compatible CPU cores like these. However if you base your server on Linux then your choice really opens up. Linux is compiled for pretty much every CPU available. PowerPC for example. CPU's that can be arranged in vastly multi CPU configurations. It always seems to me that the CPU makers have to work a lot harder to make their chips Windows compatible compared to how hard the Linux compilers have to work to support a new CPU.
Effectively with Linux (or Unix) you can simply design the most powerful CPU or cluster forgetting backward compatibility then put Linux on it. With Windows you have to design to the machine code already in existance.