i'd probably choose between the s4 or the m3..... Rear wheel drive is always fun but the s4 will come with a better spec i would of thought?
Depends how much you want to spend but there's some good examples of both out there..... for an all-rounder i personally would lean towards the audi but for weekend fun the M3 would win!
here's a couple of decent examples....
audi
http://pistonheads.com/sales/3439294.htm
m3
http://pistonheads.com/sales/3420332.htm
http://pistonheads.com/sales/3316582.htm