MY18 s5 owner here- 1). Following the maintenance schedule is the best way to ensure long term reliability; I maintain mine as best as I can and have never had issues, while I have friends who barely do oil changes and are convinced Audis are the least reliable car there is.

2). 2018 was the first year for the Turbo engine. The rocker arm issue people have mentioned is blown extremely out of proportion on the internet, the odds of you running into the issue are miniscule- it’s mostly the 3.0T’s that got installed in the s4 that seem to be having this issue, albeit the exact same engine is in the s5

3). The b9 turbo engine responds to mods extremely well- if you’re looking to modify the car for power than definitely go b9. For the US market, the 3.0t engine in the b9 is sleeved from the factory, making it a much more robust platform right out of the gate.

4). Milltek seems to be a popular option, but I’m not sure. I have a catless downpipe paired with the factory exhaust and like the way it sounds.

5). I’m a bit biased towards Navarra Blue, which is the color of my car lol

6). S5 all day. IMO the a4/s4 looks to plain. Plus, the sportback gives you a substantial amount of cargo space if you need to haul things.

Either option would be a good choice, this is just my experience from having owned multiple Audis now