I don't have the Perfection 3200. Anyway, the fact is USB2 and firewire allow the max data transfer rate of 480 Mbps and 400 Mbps respectively. USB uses more CPU resources than firewire. Nevertheless, if you got a fast CPU and lots of memory, then you shouldn't see any noticeable difference in performance between connecting thru USB2 and firewire.
P3200 is the world's first 3200dpi home use scanner. It replaced the legendary P2450. But I can't seem to find any review on the P3200 out there. Perhaps you can let us know later if it's good especially in scanning films.