They couldn't be more different, you think, right? The supercar Mercedes SLS AMG, heavily marketed and much sought after (you are wanted to think), and the Peugeot RCZ the Audi TT competitor with its hot design and acceptable price. They clearly are different!
The SLS is 6-7 times more expensive than the RCZ but has a worse drag coefficient and of course more air resistance. It's of course heavier too. It's slightly quicker in the slalom disciplines (looking at the Auto Motor und Sport test results) and can get faster into the VDA fast lane change, but comes out slower! Everything that is about economics though, the Mercedes is much worse. Everything that is about power to weight ratio or absolute power, the SLS beats the RCZ by factor 2 (almost).
The good thing is that there's barely anybody who has to decide between these two cars. Two market segments, two cars. But both cars certainly can be great fun, because fun is not a function of budget, it's more a function of expectation and engineering.