I can't comment on the AP diagonal but I have had at three different occasions three different 2' Televue Everbrites diagonals. At the same time I have also had a common or garden 2' GSO quartz diagonal. Despite costing more than three times as much as the GSO diagonal I couldn't discern any differences (ie image superiority) from the Televue Everbrites over the humble GSO.
Ironically, if anything images generally seemed slightly brighter with the quartz GSO
So, for pure optical performance the GSO quartz is the best buy under the sun (imho). The only real benefit that I can discern from the Televue Everbrite is the one piece design which provides you complete peace of mind that the barrel won't roll and unscrew if you are using any big heavy eyepieces.