The Porsche 911 is a more focused sports car and offers more interior space, but uses a fabric roof that has less security and isn't as luxurious as the Mercedes. The BMW M6 has more interior space, with somewhat usable rear seats, and a massively powerful engine, but lacks the luxury and image. The Bentley Continental GTC is even more posh and exclusive, also with rear seats, but lacks a folding hardtop and is much more expensive. And the Audi R8 is a much more aggressive sports car that is better to drive quickly, but doesn't have the same opulence as an SL63.
So the SL63 is perfect for people who want a glorious two-seat roadster that seamlessly combines power and luxury. It may not be the outright sports car competitors from Audi and Porsche are, or the spacious grand tourer, but it has a trick folding roof and loads of power to make it fun on a back road. Plus, the three-pointed star on the grille is one of the strongest style statements out there.