I don't think there is an optimal length for a BD collar. It will vary from person to person, collar to collar, etc. There are just too many variable to account for if trying to determine an optimal length. Collars, buttoned or not, just seem to require a little back and forth with the maker, followed by a little trial and error.
The roll looks a little softer on the first one, which I think most of us in this thread agree is preferable. The second just seems a bit stiff for being fairly long. However, I don't care for the double-buttons on the first, but that's more a matter of personal taste.