USB speeds will depend on your CPU speed. USB 2 is theoretically faster, but in most tests FW beats it hands down. Especially with large or streaming files. So if you have a ton of files you're transferring or if they're larger, FW is the way to go. However, to answer you're question, if you have a pretty fast PC (well, pretty much anything over a 1GHz CPU) the difference will probably be negligible. From a few seconds with small or fewer files (like 15, 5MB mp3s), to maybe a few minutes tops for larger or more files.
If you already have USB 2 it's no biggy for what you'd do, but if you have firewire or can get a firewire card cheap, I'd use that. Do a search in the forums for previous threads on the subject that can probably go into more detail.