The Dark Sky has framing errors but best pic quality.
The Retromedia disc is full frame but pic quality is less good and it is shorter...so both dvd's are of interest I guess....
The Dark Sky disc also has a featurette entitled “Dieter Eppler: Interview with the Vampire,” which is a 12-minute interview with the then 80-year-old actor.
The Retromedia dvd of Slaughter of the Vampires has a lot more heaving bosoms on display which are not visible because of the framing on the Dark Sky dvd.
It has print damage though (missing frames/footage).
Neither presentation is perfect and the framing is off on both so...I own both till a "definitive release" comes along.