If you're going into the 12-pot game, the S600 is a much better pick, as is Audi's A8L W12.

If you're staying within the 8-pot game, then it's really quite subjective, although the S550 wins in the engine department.

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2008 ML320 CDI | Iridium Silver | Black ARTICO Leather | Premium Package (H/K Logic7, Parktronic, Rear Heated Seats, Rear Power Hatch, DVD Nav, 10-way Front Seats w/ Memory) | Backup Camera | Aluminum Running Boards | 4500K MTEC Low-beam HID | 30% Metallic Tint | Boomerang2 Recovery System | Weathertech Front Floorliners | Bridgestone Blizzak DM-V1 Winter Tires