Yeah I know some mechanical keyboards can get full n key roll over via ps/2, but other than that is there really a big difference that matters? Be it for gaming or typing. Some people I know that code swear by their happy hacking pro 2 they use with ubs, others use cooler master mechanical blue via ps/2 because they want full nkey. Other than nkey does it really matter?
PS/2 is a a hardware interrupt to the computer where as USB is software. This usually does not make any difference but if you have ever had a PC bog down or a program crash hard enough that the keyboard will not respond this will override that. It is a physical interupt and waits for no operating system!
USB runs through a software handling stage where all the USB devices ask for what they want and then the software gives it to them (over simplified) but this is a problem if the aforementioned lock up happens. The USB is reasonably low down the pecking order and can be sidelined in favour of throwing the processing time at working through the lock up.
On the other hand if you are L337 G4M3 H4X0R PS/2 only runs at 250hz polling rate where as USB is 1000hz. This is also something most people won't notice but it matters to some people, like 1ms response times on monitors compared to 4ms, most of the time you will not see it but some swear the do all the time.
There might be more and people will probably be able to explain better but that is how I understand it to be.