Not to beat a dead horse, but there's no way you can be close to everything in Berlin. So someone with mobility difficulties will likely want to take taxis. I did notice at the Mercure I stayed at that there were a number of people with disabilities (more than I generally see at hotels), who no doubt appreciated the wide hallways and modern design when navigating with their wheelchairs. I can't see the Westin being so user-friendly for wheelchairs, but I suspect the Melia is, being a new hotel (again, I haven't visited, because it wasn't open when I was there).