I am making a build with a 1TB hard drive and a 64GB SSD hard drive. Which one should I install Windows 8 on and why?
The SSD, the OS will boot faster.
You're probably going to want more than 64GB on the drive that you have your OS on, especially Windows 8.1. As updates come in, Windows will require more and more space to operate. A lot of people are already a little upset about it because they feel that Windows is already taking up too much space on their 120GB SSD.
I would just put a few of your favorite games on a 64GB SSD.