If you need the drive to be able to boot those Macs, you'll need FireWire connectivity. If you ned to use a system optical disk to upgrade or re-install the OS. or repair your hard drive, you must boot from the optical drive. If either computer's built-in optical drive has failed, or does not support reading DVDs, you'l need to boot from an external sooner or later.
If all you need is a burner, then USB opticals are cheaper and should work fine.
This firm tests all their stuff for Mac compatibility: