Hi Wkerat
I am lucky enough to own both a 04 S4 Avant Tiptronic and a 04 M3 Coupe. As has been said earlier in this thread it is not a choice between the cars, but a choice as to what you want out of the car itself. As you can gather I have both because I want what they each have to offer. Try this on for size.
S4
+
Grunty engine note
2 dogs in the back
Build quality and class
4 Wheel drive (need it in scotlands recent weather)
Easy to drive hard
-
Vague-ish steering - does not translate everything from the road(maybe a + on long journeys)
Non-body coloured side strips
M3
+
Engine note - sounds like a race car, really urgent
Absolute precision steering - point and shoot
Driving anywhere is a pleasure, foot on gas = constant grin
SMG II - Sequential Manual Gearbox thingy - techincally awesome
Awesome colours/interior trim available(mine is an individual colour combination - lurvely)
Nice touches (auto lock doors, single door unlock - as std)
-
Interior look more practical and not as classy as AUDI
I'm not driving it right now!!
Hope this helps, if you need anything else including, dare I say it, a great M3 forum then just let me know.
Have fun on the test drive(s)