If your dad is cross-shopping a big Mercedes luxury sedan and a high-end Porsche 911 and nothing else, then I would seriously doubt his enthusiasm for incredible all-around performance and suggest that he get the Mercedes. Most people who are in the market for a 911 Turbo or 911 GT3 are either rich car collectors or serious drivers who are also looking at a Ferrari F430 and Lamborghini Gallardo, among other cars. I don't know your dad obviously, but based on what you've said, I would imagine he would either baby it everyday and use the car to 10% of its full potential or end up plowing it into a freeway center divider. The 911 Turbo and GT3 aren't toys, they're serious machines that will punish you if you screw around, especially the GT3. But if he still wants a Porsche, have him test out the new Cayman S, those things put out really awesome numbers and have the potential of overtaking a portion of the 911's customers.