If memory serves me right, then using USB over PS/2, is actually a step back for mice. PS/2 has a higher refresh rate than USB.
Personally, i`d say use the USB port, i do. If the MX700 cant do over the max for USB, then why move to PS/2 ? Theres no way that the rate can be increased.
People will argue that the USB port uses more CPU time than PS/2, but with todays CPUs, the performance loss isnt noticable. your talking about 0.00001 of a frame, in games.