I'm curious, why you want to use USB?
using USB would be unnecessarily complex... PS/2 is a very simple data transmission scheme (similar in complexity to synchronous RS232) and is only intended for keyboards and mice, so it's a simple job to decode it... and there's really no overhead, because it's pretty much just direct serial data. (my routines for PS/2 communication on a PIC are some pretty simple C code, nothing very fancy at all) not to mention, you can easily find all the information you would ever need on PS/2 communication in just a couple minutes of google searches.
with a USB mouse, it would (presumably) interface with a PC appearing as a human interface device, which the computer would then recognize as a mouse and deal with accordingly... however, to actually use one in your project, you would have to deal with all the overhead of the USB communication, including recognizing the device, configuring things, and then maintaining USB communications. the actual data transmission would probably be just a small part of the complexity of the communications link.
not to mention, you'd probably have a harder time finding information on interfacing a USB device with a microcontroller, than having a microcontroller appear as a USB device to a computer. if you are curious, try searching google for using a PIC as a HID (human interface device) on the USB port and see how much work is involved...
Bear in mind that USB mice can generally be used as PS/2 mice as well (with those little usb-ps2 adapter plugs that they usually come with), and I would imagine those are nothing but wiring inside... so presumably the mouse detects from the state of the data/clock lines whether it is connected to a USB or a PS/2 port, and sets its communication mode accordingly. in this case, even if you wished to use a USB mouse simply because they're becoming more common, you should still be able to use it without having to deal with the headache of USB communications.
*edit: here's the two pages that were pretty much everything I needed to know for doing my PS/2 communication routines:*
one describing the actual communication protocol:
**broken link removed**
and one describing the data that the mouse sends, and how to control it:
**broken link removed**