Gary,
Like you, I was confused about the 2 rear fender flare options when I ordered my ERA. From their website I was under the impression that the difference between the two was essentially how wide the lip was. That is, how far did it protrude out from main body. When I went to ERA, they showed me the difference and from the outside of the car there was no difference to my eye. The difference could be felt though when you felt the edge of the flare. The edge of the optional flare was much thicker and felt kind of like it was rolled under. In summary, I'd say the difference is the thickness of the rear fender flare edge rather than how much it flares out. I did end up going with the thicker optional flare. Pictures in my gallery but you can't see much from the photo.
Chris
http://www.clubcobra.com/photopost/s...0&ppuser=11198