Opinion sought. Key/Mse con USB or PS2 best?

Hi there, I am trying to get some advice from one knowledgable in connecting computers via KVM.
Is a USB keyboard/mouse connection to a MS server box as good as or better than a traditional PS2 connection?
I will be using a Belkin KVM over IP unit over CAT6 backbone 150m to a control room remotley accessing a IBM Server and other XP building managment boxes.
My experience of a USB connection (using a 2 port USB KVM,Local only) is that there is a time lag on mouse control when I switch between computers.
Also my concern of full control of the server as well as speed and video quality?
Im rambling on now
So if you can help
Thanks