"What is the benefit of the much more expensive 'Time Machine' or true Apple external drive?"
There are none. Absolutely none. In fact, many people have had serious problems with Apple's "Time Capsule" due to overheating issues because of the poor design of the case (no provisions for adequate airflow and cooling). Buy one of these at your own peril.
"Would like to get a firewire external hard drive for backups only."
My advice goes in two directions:
FIRST WAY:
Get yourself a good external enclosure and a "bare" hard drive (manufacturer and size of your own choosing).
Then, assemble the drive into the enclosure. This will take only a few minutes and maybe a screwdriver.
The reason you want to do it this way (rather than buying a pre-assembled external drive from any particular manufacturer) is that by doing this simple job yourself, you will have learned "how the thing goes together" and more importantly, HOW TO TAKE IT APART if there is ever a problem with the drive or the enclosure. You won't be intimidated by the "innards and workings" of the drive -- you will know what's going on with it.
It _will_ probably cost you a few bucks more to do it this way. But one other advantage is, if you buy a pre-assembled drive, it usually comes with only a 1-year warranty. If you buy a bare drive (newegg.com is my favorite source), it usually comes with a 3-year warranty, and some drives come with 5 year warranties.
Here are a couple of enclosures I don't think you can go wrong with:
http://www.amazon.com/Macally-G-S350SUA-Hi-Speed-FireWire-Enclosure/dp/B000P1NAMO/ref=pd_cp_e_1
http://www.amazon.com/Macally-Hi-Sp...3?s=electronics&ie=UTF8&qid=1294327536&sr=1-3
The first is firewire400 and cheaper. If you REALLY want the extra speed of firewire800, you can pay the extra $$$ for the second (it has USB2, firewire400, firewire800, and eSATA). It's your choice.
SECOND WAY:
Don't buy an external "enclosure" at all. Instead, get one of these:
http://eshop.macsales.com/item/Newer+Technology/FWU2ES2HDK/
And again, buy a bare drive of your choice. For extra protection, get TWO bare drives. Then do TWO "clones" of your internal -- one to each drive.
Rotate the backups as desired. If you have a secure place "offsite" (different building than where the computer is) where you can store the second drive, even better.
The advantage of using a "dock" is that the drive isn't "tied to" any particular enclosure. Also, you have a quick-n-easy way to connect almost any drive you encounter to the computer. A VERY useful tool to have around.
FINAL THOUGHTS:
DO NOT use Apple's Time Machine to do your backups UNLESS you know exactly why you need it.
Instead, use either CarbonCopyCloner or SuperDuper to create bootable clones of the internal drive to the backup drive.
You will not recognize the importance of having a BOOTABLE backup drive, until the day comes that you have something wrong with the internal one!