2010 Mercedes-Benz E550 Sport vs. 2009 BMW 550i Sport vs. 2010 Jaguar XF Premium @ INSIDELINE.COM: www.edmunds.com Building an ideal executive-style luxury sedan is not an easy task for a carmaker. If the car turns out too soft and isolated, you lose enthusiasts who aren’t quite ready to retire in a Lexus. Too complex or high-strung and you risk alienating your technology-challenged, soft-in-the-middle buyers who expect a measure of mindless comfort. Get the balance just right, and your five …