being that i just bought a 2002 M5 last week and previously owned 2 B6 S4's (both manual) i can only tell you that its 2 different cars. The M5 has way more room inside and is more comfortable while the Quattro on the S4 is amazing (especially if you drive the car in the winter) The S4 is a great car but it has 2 major problem Chain Tensioner failure and excessive oil burning (my second S4 was burning a quart every 300 miles) aside from that its a solid car that sounds AMAZING. The M5 on the other had (early production 99-00) you will have issues with the VANOS oil consuption and a lot of blow by, so my suggestion is to look for a 01-03 plus those cars have nices headlights/tails and better looking interiors. At the end of the day the S4 is an awsome car but the M5 is def upgrade
Down and Out that's the way to go...