Polling rate shouldn't matter with an ipac. (feel free to correct me if wrong).
In my experience (as I have used both with my ipac) the only difference is USB is plug and play, and PS2 is not. Meaning, if you unplug, or try to plug in the PS2 for the Ipac while the system is on, you will have to reboot.
This is the only difference I have found (albeit there may be some micro unnoticeable other effect I am unaware of).
Personally I use the PS2 so it leaves more USB slots for other items ...