In a road car I would have either. The viscous diff is good in day to day use. As long as it does not get too hot they work fine . (I have one in my E3). For competition or hard road use a clutch pack(mechanical) is the way to go. Later this year I will be doing some club sprints and will most likley change to mechanical. At the moment the car is a daily driver that occasionally has a bit of a hard time on a winding road. The viscous is fine.