I drive a 2010 Audi S4 with 92k miles. Been lovely, though somewhat boring, getting long in the tooth. In the market for a new sports sedan (kiddo on way), so I test drove a new M3, C63, and S4 today. I realize the S4 is not comparable, but hear me out. I would be comfortable spending up to $85k and this was my first time driving these cars. I used to own two BMWs (E46 M3) and my impressions were quite surprising...
BMW M3 -- Looks the best, felt "tightest" in a good way, was aggressive and loud with ZCP (though loud doesn't mean great-sounding) ... but the interior (this one didn't have full leather) felt meh ... dated, boring, very 2013-ish. I wasn't completely blown away for $80k+.
Mercedes C63 -- Would have driven the S if they had one. By far the best engine and exhaust, obviously. Interior ... looks great from a distance, but when you are in it, you see hard plastics and cheapness. Some of the buttons by the gear-shifter felt cheap. My biggest gripe was the cheap hard plastic material my right leg would rest on while driving -- yuck! Also hated the COMMAND system and the nav screen looks ugly. Lastly, the panoramic sunroof has a thin screen when closed that's completely translucent. I hate that, when I have it closed I want to block out the sun. Overall, love the sounds and power ... that's it.
Audi S4 -- They had a pretty loaded Premium Plus with 19s. Yes, it looks less aggressive than the others, but still very sharp. Immediately the cold start exhaust in the showroom impressed me, it sounded much better than the old S4. Not M3-loud, but aggressive. Interior feels a LEAGUE above the others ... fresh, updated, all that new technology is awesome. And I kind of like the seat massagers for a DD. Diamond-stitched leather, alcantara, flat-bottom steering wheel, just felt RIGHT when sitting in it. Driving impressions: feels a step faster than my old S4, and the biggest change is the exhaust is WAY nicer and louder, which was my biggest gripe before.
Overall impressions -- The BMW and MB are faster and $20k more expensive. But my gut reaction was are these really necessary? They feel like overkill for a DD, and the S4 is so much more practical and yet still fast, fun to drive, and luxurious. It feels like a great car without trying too hard or being too ostentatious. I am seriously leaning towards the Audi (with Black Optics). I wasn't even going to look at Audi because no RS4, but I'm glad I did. Major thumbs up. Oh: and I can get a 2018 custom-ordered S4 in 60-90 days, whereas the others don't even have ordering guides out for 2018, so if I wanted the M3, I wouldn't be getting it until September.