The best limited slip diff is the Quaife Torsen (ATB) unit. There were/are some ZF units still about but IMO the Torsen unit is much more durable and gives a softer take up that is more controllable in less experienced hands. Porsche on the other hand preferred to use the ZF plate diff on their 934/5 racers for a more positive lock up but then they eventually ran fully welded diffs!
http://www.quaife.co.uk/shop_categor...tegory_id/1274
Clive