I really like look of the long wheel base better. Always drove long wheel base S-Class Mercedes' before which also look nicer in its stretched version.
What is different is that, subjectively, the rear door on the BMW look so much larger than on the MB. That emphasizes the longer version IMO.
I think that's due to the fact that the window is larger and the front door seems a bit shorter. At least I feel a whole lot more B pillar next to me than on the MB.