The C63 is a beast but to be honest, sometimes I see a regular C-class with the bodyworks and its hard to tell if its an C63 or not (only time I definitely know is when I see the dual tailpipes from behind). I think part of paying the premium of a car like the C63 is to make it stand above the regular C-class cars (like BMW does with the M vs 3-series)

 

I had a C55 as well as my Porsche for a while, and loved them both. The decision should be driven more by how you intend to use the car and what features you need or want. The AMG is great if you need room for 4, a nice big trunk, and a quiet cruiser that goes like stink. If you don't need the room or the quiet, and enjoy the engagement of a stick shift and better cornering, then the Porsche wins. I sold my C55 last fall, but there are days I wish I could take it for a run again. Picking one based on depreciation is less important. They will both cost you a lot, especially if purchased new. I've only purchased 1-year old cars over the past 10 years and this has helped cut my (large) losses in depreciation. I would feel very nervous owning an AMG after the warranty expires.