Hi Anders,
Firstly I should say I am 100% biased because I own an S4 B8, as for which one would suit you best it very much depends on what you need from the car, the 3.0TDI will offer the best economy but the S4 B8 will put smile on your face, also it has huge reserves of power for acceleration which also makes it super safe when overtaking. I went for the DSG because the 7 speed auto box best suits the engine, the way the car drives is hard to explain and you really should take one on a test drive, I think it has more pull then the diesel and you can rev it past 7,000revs / minute. Both I think have the 4th generation Quattro which is a permanent 4 wheel drive system, driven the car in heavy snow with winter tyres and I was overtaking all the other cars on the road including Land Rovers.
When I decided to get a 2nd hand car I did invest in a code / diagnostic hand held tool so I could check out the car, saved me time and money.
One known issue with the S4 B8 is the steering rack, solved on the 8.5 I am told but I had to replace mine because there was excessive movement within the rack, it made a world of difference. It is important that work is carried out at the correct time/millage, I got lucky with my car and I have a copy of all the repairs/service work carried out since new.
Now that I no longer use Audi main dealers my car is super reliable, the garage I use does a great job of looking after the car and has guided me on what upgrades to consider. Last summer I drove from the UK to Denmark in one day with 0 issues and that included testing the max speed on the German autobahns 🙂 (200mph+) then went via Sweden and Norway through the mountains into the arctic circle. My car has about 125,000miles on the clock and I have been told the interior looks like a car that's has done 30,000miles, the only real money I spend on the car is the performance upgrades, servicing is not very expensive.
Have carried out a number of upgrades on my car, so if have any interest in getting the S4 B8 so you can upgrade it, myself and others can give you the pro's / con's on them.
All the best
Edward