From the OCN Guide
PS/2 or USB?
PS/2 wins on three fronts: First, it supports full n-key rollover. Second, PS/2 keyboards aren't polled, but are completely interrupt based. And third, it is impossible for it to be delayed by the USB bus being used by other devices. There are two types of USB transfer modes - the interrupt transfer mode (USB polls keyboard, when key is sensed the USB controller sends the interrupt to the CPU), and the isochronous transfer mode, which reserves a certain amount of bandwidth for the keyboard with a guaranteed latency on the bus. Unfortunately, there are absolutely no keyboards made that use the latter, because special controllers would have to be used, thus making it cost prohibitive.
From the HardForum Mechanical Keyboard Guide
When purchasing a mechanical keyboard always choose the PS/2 version over the USB version. PS/2 offers infinite N-Key rollover while USB only offers 6 key rollover with 4 modifiers. If you need a USB connector for your PC, purchase an "Active" PS/2 to USB converter. Non-active PS/2 converters do not properly transmit all data from mechanical keyboards. You can find one here (Used with the IBM Model M Keyboards):
I would never get a PS/2 keyboard if given the choice of USB or PS/2. It's 2010 people!!!
I don't get the whole polling/interrupt argument when it's the OS/Application/Internet doing 99.99% of the latency damage. 6KRO is fine unless you are into Asian two handed music games
And that theoretical Isochronous Transfer mode was fascinating but irrelevant.Edited by ripster - 9/20/10 at 8:43am