Depends on what keyboard you have, if you have a decent keyboard with full n key rollover (nKRO) and not any gimmicky lights the ps2 is the better choice, usb is limited to something like 6-8 KRO.
Also I've heard ps2 is more direct with the cpu instructions get fed straight to it rather than going through a bus that can get busy which will cause the instructions to lag.
I run ps2 with a filco mechanical keyboard, it is amazingly resposive and I can press every single button together.