Jim Larson , Dec 21, 2004; 12:09 p.m.
I have not used both systems extensively, but I have researched enough to comment on a few of your questions.
1) I can't comment on "quality", as I have not shot E-TTL II more than a few times (with a film body).
2) A given camera body will use either E-TTL or E-TTL II exclusiviely. This is firmware/hardware configuration issue and is not user selectable. Kind of like if your camera has partial metering or a spot meter -> you get one or the other and that's the end of it.
There are no firmware upgrades to convert a 10D or Elan 7 to use ETTL-II.
3) E-TTL II refers to the *entire system* of flash metering. This system includes a correction factor for lens focus distance. This correction factor is not used in the conditions you describe (ie, bounce flash). This correction factor is also not used if you use a lens that does not provide distance feedback to the body.
I do not know the importance of this correction factor, but *I suspect* it is minor.
The primary difference between E-TTL and E-TTL II is that E-TTL is strongly focus point biased. ETTL evaluates the flash exposure primarily at the focus point. E-TTL II does not have this high focus point biasing, and the flash exposure is calculated using more of an evaluative metering pattern.
The primary benefit of E-TTL II is that you are no longer so reliant on Flash Exposure Lock to get correct exposures. Much better for shoot-on-the fly photography. E-TTL works very well in slow motion situations, where you can 1)press one button to do FEL, 2) press another button to focus and 3) mash the shutter to take the image.