One difference is that some truly ancient systems (with PS2 ports) don't know to 'look' for USB keyboards. On those ancient systems you first need a PS2 keyboard to boot the system and install the OS or work in the BIOS.
i saw a video on youtube where a guy was claiming that if you connect your keyboard through a USB port, it will allow a "6 key rollover" while if you connect it through a ps2 port, it has "infinite key rollover". He was using a das mechanical keyboard. I was wondering if that was true for other keyboards as well. Thanks