I have owned the 911 997 and now have the SL63 2008. I also owed the 55. But my choice is the 63
Both vehicles could not be different. The Porsche is a much better cornering car.
The 63 is just a super fast car in a straight line and unlike the Porsche will have you with a smile on your face from the great 6-2 engine.
Running cost for me the Porsche is cheaper but that is because the Porsche is serviced every other year.
Having said that neither has let me down.
The Merc has a better interior than the Porsche bland interior . The 63 has better space available for storage too.
Both are cheap on insurance but both have just under £600 road tax a year.
I may decide to put mine up for sale but will only buy a newer version if the right model become available I have only covered 53k miles and do minimum miles each year.
I find it alway best to allow all high powered AMG to allow the oil to heat currently before driving with any real revs.