I had an S4 B6 avant nogaro blue for one and an half year and it was a great car , did look really good in that color and i had a supersprint exhaust ,the sound was fantastic. Very practical, plenty of room, well build, very strong engine but very firsty

I really did love it. you can get one now with low mileage for very cheap and i do not think they will drop on price dramaticly in next 2,3 years.
I did switch to an RS4 b5 witch is smaller, leg room on the back is rubbish, they look a little bit dated now, BUT they are mega fast and will give you a lot of satisfaction
The S4 is fast as well but feel a little bit heavy compare to the B5 RS
All depend on what you are looking for....
To me both are great cars, wish i did keep the S4 would have saved a lot of money...very tempting to do mods on the rs!!!!!!!!
Porsche 996 Turbo S Atlas Grey.
RS4 B7 Avant Sprint Blue. Sold.
RS4 B5 Avus Silver with MRC magic . Sold (Best car ever )
S4 B6 Avant Nogaro blue. Sold
A6 Avant Silver . 2.5 TDI .Sold