There's little point in discussing the original firmware 476 since, to the best of my knowledge, that cannot be "rolled back" to, and only the very oldest Gen3's have it.
There's a very comprehensive discussion of what was fixed between 476 and 538 (the version which most "First Generation Hardware" Gen3's have) here
The "Second Generation Hardware" (ie 512MB RAM, improved nav button) Gen3's ship with firmware version 674. That appears to be identical in fuctionality to the "upgrade" version, 685, other than displaying the firmware version as "1.0" rather than "1.1". Once you reflash a "2nd gen" machine to 685, I don't believe there's any way to return it to v674.
Really, therefore, the only question that arises is for owners of "1st gen" Gen3's - should they reflash to firmware 685 or not? Personally I found it a minor, but worthwhile, upgrade, but I don't use PDFs (which appear to have been rather "broken" in the upgrade).