The choices that you gave are of cars that are meant for completely different purposes. A better set-up IMO would be a MB S-63 AMG and and the Quattroporte, or the SL55 and the new Maserati GranTurismo. To answer your question, out of the two you mentioned I would take the Maserati Quattroporte.
QP all the way. More style. More character. Why look like every other poser on the planet by driving a Benz?
Two different cars because one is a two door and one is a four. If you do not need the four door, go with the SL55 but for a four door, the QP is amazing!!
personally, i wouldn't drive either. there are too many benzes in greenwich as it is, and the nightmare i forsee with owning a maserati is too much to bear. i'm more into the audi S8
The QP will not hold up over time.... We repossessed a QP from a 62 year old man about 4 weeks ago, it was one of his vehicles. He refused to pay for it because he had warranty issues and when I drove it I was very un-impressed. I have had many Mercedes..... I just sold a CL55 AMG, the AMG's are great but lose value if you plan on putting a ton of mileage on them..... I would go with a Bentley GT, 04/05, they have come down in price and are a very driveable car, I just sold mine about 6 weeks ago, making room for the Flying Spur..... Seth