Pink does more than you think.
The Mercedes S-Class is a great car, and you get what you pay for, through and through. Mercedes is pretty much 100% luxury, which makes the interior all business. The 7-Series interior provides awesome luxury, with some sporting intent, but not nearly as much as the other BMW's as it is a super-luxury sedan. Interior quality is pretty much the same for both cars. Being inside both cars, I think I like 7-Series better as it offers a more entertaining drive, as well as the interior design being more focused on driving.
If you want a really great combination of drive and luxury, the Audi A8 L is a really great car. My friend's dad's A8 L W12 has the nicest front seats I've ever been in. It is unbelievably comfortable while holding you in place sooo well during sporty driving.
In general, the Germans know how to make these huge beasts feel smaller than they really are, and drive better than you think they ever would. None of the cars is a wrong choice and they all offer a blend of luxury and sport, but they each have their purpose: the BMW is the best for sport, the Mercedes is the best for luxury, and the Audi is the best in between. Also, all three vehicles are extremely practical and easy to drive, and make for great lifestyle cars.
MG, I agree that the X5 interior is more about sport, but everything is so well-integrated in that car. The quality is outstanding, and good use of wood trim and premium interior materials, as well as great access and layout of interior controls, make the X5 interior a really nice place to be. The X5 benefits from being a newer and more expensive SUV than the previous-generation ML (the one you have), but the ML is much more well-rounded. You get much more trunk space, low-range gearing, and a drive that is well-taylored to on and off-road driving.