[RWebb]

Ok, here is a quote I found using Goggle:

Over the last several years Audi has started calling both their all wheel drive systems 'Quattro'. The first system is made by a company called Haldex and it is found in the Audi TT - this system is basically front wheel drive (90/10 front/rear split of torque) until slippage is detected and it then will direct more power to the rear as needed.

Audi's second system is the traditional Quattro system that you find in the A4, A6, A8, etc. It is technically complex with 3 different differentials - a center torque sensing differential to direct power front and rear as needed and front and rear electronic differentials to direct power left/right as needed. The real Quattro system is very good at providing traction where it is needed.

Subaru also has severl AWD systems. The first system, Active AWD found in most of their automatic transmission equipped cars (e.g. non-turbo Legacy/Outback) and it operates very similar to the Haldex system. The second system is called Continuous AWD. This system is found in most of Subaru's manual transmission cars. It uses a mechanical viscous center differential to split power 50/50 front and rear. Once slippage is detected, the system will react and send power to the wheels with traction. Subaru also uses the VTD (Variable Torque Distribution) system, which is closely related to Audi's Quattro system as it uses electronics to vary the power front/rear. I believe this system is found in higher end Legacy/Outback models. The STI uses a version of this system but has both front and rear limited slip differentials as well as the ability for the driver to select how much power to send front/back.

All the systems are very good with the Quattro probably being the 'best' albeit the most complex as well. The Continuous AWD system that Subaru uses is very simple and probably the most bomb-proof.