Get the larger capacity USB drive. As has been mentioned previously, Time Machine will generally be transferring more slowly than USB or Firewire permits, simply because there are delays in disk activity when going through multiple files.
Firewire is good for applications requiring a fast connection to the drive. For example, if you were doing video editing and had your video stored on external drives, you would want Firewire (eSATA would be even better). That's not a case of copying files, but of actively accessing large files. Program responsiveness and such depends on how fast it can read and write the data.