BITE YOUR TONGUE!!!
PS/2 is greater than all! All these youngens these days putting flashy lights as more important than latency and proper N key rollover.
depends what you want. Both USB and PS2 have their advantage:
Speed:
The fastest connection is PS2 simply because it has a direct interrupt request link to the cpu, so the cpu quickly gets notified when new inputs are incomming.
Due to the Bus archtitekture of USB it may take few milliseconds longer until the CPU get's the message.
Utility/ Error-Korrektion
The Problem with PS/2 however is, that the number of keys that can be pressed at the same time without the appearence of ghost-keying (A key that didn't got pressed is submitted as pressed) is havily limited.
USB-gaming-keyboards solves this problem, as it signsup as multiple Keyboards.
By doing so they can accurately identify 8 Key strokes at the same time.
So if you're one of these guys buttonsmashing all day, you're propaly better off, with a usb-keyboard. And if these keys are often around the w-a-s-d- keys, you may even want a special gaming keyboard, as these are optimised to have as little ghosting around these keys as possible.
But if you're more the twitchy LoL gamer, a nice PS/2 may give you that millisecond advantage to steal the grunt kill from your mate...