I don't see any difference in speed between external and internal burners. Instead of buying an external drive, get yourself an external enclosure for 5-1/2" device (look at places like
www.newegg.com) and then buy a regular internal burner. It's very simple to install the burner into these enclosure, and the beauty of it is that you will be paying a lot less money than if you had bought an external drive.
There's a couple of extra bonus to putting together your own external drive. First, is that if you happen to change your mind about the external drive later, you can pop the drive out of the enclosure and pop it into your system. Second, if you have another IDE device, like another internal hard drive, that you may want to have a temporary access to, you can take apart one of these enclosure and remove the burner, replacing it with the new device in a couple of minutes.
Why are external drives popular? Portability. That's the key. One drive can be shared with many computers.
From what I had read, Firewire is supposed to have a better throughput than USB 2, IF you are transfering large files. In real life, I have 2 external (200 gigs) hard drive hooked up to my laptop via Firewire, and an NEC 3500 via USB 2 in an external enclosure, I don't see much difference in speed. I do have occasional problems with conflicts between the Firewire and USB 2 drives though, but that's probably attrributable to the drivers. Point is that I would recommend using either all Firewire, or all USB 2 for drives.
What is this life, if full of cares...