I have both and to my eyes they are excellent diagonals. The 2" quartz has both the 2" slip in barrel and 2" SCT fitting whereas the 2" Dura-Bright cannot be fitted with the 2" SCT fitting.
If you read the following links I think your question may be answered :
http://www.williamoptics.com/diagona...l_features.php
http://www.williamoptics.com/diagona...5_features.php