My money is on the 750Li. Sure the S550 is all-new with all kinds of gadgets and gizmos and it's got more power than the big bimmer. But the 750Li has the total package in terms of pure driving experience that the Benz can't match. Not to mention the bimmer looks a whole lot better and more harmonious (especially the butt) after the facelift.
Oh and one more thing did I mention the big 7 is now being widely imitated.