A Quaife is an LSD. As you said it uses gears. The ZF uses clutches. I was told by Rennsport (who is building my 915), a ZF style diff is better than a Quaife in a rear drive car for longevity and performance. The gentleman I spoke with has a 911 w/ a ZF style (Guard Transmission) LSD and a Quaife in his GTI. Go to:
www.rennsportsystems.com They will be able to explain better than I. What trans. are you installing??