Well, the S5 being a coupe is more sporty and as most coupes, it looks much better than its sedan counterpart. It's lower and has a wider track, so it handles better. Wider track = more grip when cornering and lower means lower center of gravity, so less roll and better handling as well. It's harder to get in and out of the coupe obviously, but once in, there's nothing inherently less comfortable. There is less headroom due to the lower sloped roof line, so if you are a tall fellow you may need the additional headroom a sedan provides. Also the doors are longer, so it can be tough to get in and out in narrow spaces.
The S4 being a sedan is obviously more practical if you have a need to haul people around. 4 doors makes it easier for the rear passengers and there is more legroom in the rear. Front doors are shorter, so it is easier to get in and out in narrow spaces.
At the end of the day it comes down to this. You are either in the market for a sport sedan, because you need the practicality and room it provides or you are in the market for a sport coupe, because you want something that looks and feels more sporty and you are not in the business of hauling more than one passenger most of the time.
Last edited by superswiss; 07-25-2014 at 12:11 AM.