Correction - AFAIK - if you order PS/2, you only get PS/2. If you order USB, you get an extra cable that can be used as USB as well as the PS/2 cable.
Sounds like you just need the PS/2 version, but you might pay the $5 extra for the USB cable in case you upgrade down the line and don't have to order it again.
USB BIOS support does not mean that a USB keyboard will work well in DOS, AFAIK.