Does GLS550 have anything that GLS450 can't have with options?
 

Engine aside, obviously, does it have any built in improvements over the GLS450 model? I've read before that it has double pane glass, that GLS450 does not... is there anything else that is inherently different with them that you can't get through options?

The body panels (front fascia), rims (at least not available as options in the US).

In the US, the AMG appearance package is not available on any model other than the 550. It includes the front bumper, the wheel arche extensions, the rear bumper and the AMG wheels. Real bummer in my opinion as I think it really changes the overall look of the car.

Maybe the biggest difference between a 450 and a 550 is the 4.7L V8 biturbo engine available only in the 550. There is nothing like a V8.