Unix or Windows

I am in environment having 700+ users accessing different applications purchasing, accounting, HR, etc..(Oracle based. I came up to be the decision maker for having my operating system to be UNIX operating system or windows operating system in terms of the following:
– cost (long run)
– Scalability
– Response
– Stability
– HW
– SW

Appreciating all the IT gurus to get back on the above..