Based on the following experience we have had with our 2013 ML350 and the Rattling/Creaking noises reported in this forum,I have serious reservations about purchasing or leasing a 2018 C43 AMG.
62,767 KM (Approx 37,660 Miles)
Purchased as a CPO with 16,051 KM in 2014.
Tail Light Corroded - $737,00
Air Ride-4 Level Links seized - $476.00
Lane Assist inoperative - Warranty
Door Wood Grain Trim Cracked (known problem per Service Manager) - $624.00
Front Brakes Pads & Rotors - $1,422.00 (replaced when purchased)
Left Rear Strut (Faulty Damping Valve) - $1,422.00
Only the Lane Assist was covered under CPO warranty.
This vehicle is driven by my wife and has never been abused by us.
Who knows about previous owner.
I have had zero problems with my 2012 Audi A6.
I looked at the 2018 Audi S5 SB but vehicle is too small.
Will probably wait until new Audi A6 comes out and will decide then but in the mean time will take a C43 AMG Sedan for a test drive.

The c43 is not comparable to the s5 sportback in terms of size? rear seat primarily the issue?

b9 s4 is a great car too. l I too thought my previous Audi was much better built than my current c class.

Audi S5 SB is a lot lower so I have trouble getting in and out.
AMG 43 bigger than 2018 S4 Sedan.
Following dimensions taken from Can & Driver:
Head Room AMG-40.9", S4-38.4
Leg Room AMG-41.7", S4-41.3"
Height AMG-56.5", S4-55.7"
Took the AMG for a test drive today and although smaller than my current A6, I was comfortable and could get in and out no problem. (I am very tall)
I will try the S4 next week.