It all depends on what you are going to do with the stored data. IF you want to put it on a shelf, or into a safe, and have many GB's to back up, a dvd-rw could be a good solution. You can also share the data on the dvd's with systems without firewire, depending on how you set up the disc. You could make it cross platform and share data with friends on peecee's with the dvd.
Depending on which firewire drive, it could be faster, but will probably be larger. Also, when you want to access any of the files on the drive, you have to connect it to your system, where with the dvd, all you need to do is put the disc into your reading drive. Of course, that is provided that you have at least a dvd-rom inside your computer, if not a combo drive.
You have to decide which is a better option for you, a lot storage all at once, or the possibility of having even more storage with only needing to get more blank media. At ~4.5GB a disc, you can easily get into the hundreds of GB.