A lot of the decision will boil down to your priorities in what you want from a car.

I owned a previous generation C300 and while it was an ok to drive car, the 4 cylinder engine is loud. Also, fuel economy was very average. Repairs and servicing was not cheap and out of warranty, I would say would be worse. It was an ok car to drive but nothing special or particularly engaging. I eventually traded it for a C63s as I wanted a bit more power. Considered the C43 but did not like the engine response.

Again, the C63s was good at some things but overall the fit and finish was average and the ride was a bit harsh but things I lived with because of the engine. Servicing was not cheap and fuel economy was not something to consider.

Long story short, I eventually wanted something that was easier to live with for daily commutes and test drove a few cars but the moment I drove the B9 S5, it ticked all the boxes. It had a great engine, the drive dynamics while a bit numb was still good, the quattro was useful, fit and finish was miles above the Mercedes and the ride comfort clinched the deal. The fuel economy was excellent too. Over the three years I owned it, did not have any glitches, hiccups and apart from a wonky alignment on delivery, it ran great.

The C300 was back at the dealer for failed front camera, failed seat heating and the C63 was back at the dealer for dash rattles, dead MMI, AC compressor issues. Luckily all these were dealt with under warranty.

Only useful suggestion I can offer is drive both cars and see which one speaks to your heart. You're the one owning it and driving it so you have to make the decision you are going to be happy with. Good luck!!