2000 or XP

I have 17 machines that I am about to get licencing for and I would like to know some experienced opinions on which OS I should get for them. They are all PIII 600MHz. Two of them have 20GB HDD while the rest have only 10GB. 10 of them have 256 PC133 Ram, while the rest have only 128. I know they meet the requirements for both, but which would run faster?