PS2 has the ability for higher polling interval BUT if you
don't need that higher interval, or left it at defaults, USB
might be as fast.
Since mice are typically only USB1.1 devices, plugging one
into a USB2 port may make other devices connected to same
port pair also work at USB1.1 speed, where it might've at
USB2 without mouse.
If you find yourself often unplugging/replugging mouse for
(whatever) reason or had a KVM with particular port needs,
that might weight in favor of USB, but otherwise you're as
well off using PS2 instead.