I would be inclined to favor PS/2, on the grounds that it doesn't use up a perfectly good USB port, and it would work on older computers. The fact that USB is limited to 6-key rollover would not have affected my decision much, and I don't use hibernate or standby since too much software is not compatible with those modes.
But newer computers may eventually be "legacy-free" and not support a PS/2 keyboard port. There's the safety issue of not having to worry about blowing the motherboard if the connector is jiggled.
And, at least potentially, there could be a USB port on the USB keyboard to plug the mouse into, following what Apple did back with its earlier ADB port, which nicely reduces cord clutter. That, to me, is the biggest argument for USB.
So it's very much a personal choice, based on the factors people have cited here.
Since my favorite type of keyboard among the keyboards I own is the Model M, in choosing a computer I would tend to make sure it has a PS/2 keyboard port, but that isn't the same thing as choosing the type of interface when buying a new keyboard.