I'll be flying into Schoenefeld Airport and travelling to a flat in Hessischer Strasse. Two public transport options: S-Bahn to Hauptbahnhof and then bus to Hessischer Strasse, or S-Bahn to Friedrichstrasse and U-Bahn to Naturkundemuseum. I will have heavy luggage and want to avoid stairs/escalators as much as possible: which route has less?
Philip, checking both Schoenefeld and Hauptbahnhof: You can, as you said, take the Airport Express to Hauptbahnhof. On the Hauptbahnhof web site it says, that, due to construction, there is a longer walk to the bus stops. On the map, it looks like Hessische Strasse is just a little over a kilometer from the Hauptbahnhof. It might be worthwhile taking a taxi to the door.
Philip, From Berlin Schönefeld Airport is a special train "Berlin Schönefeld Airporter" that goes to Hauptbahnhof. where you can take the S-Bahn (west) to Bahnhof Friedrichstraße. Both Berlin Hbf. and Bahnhof Friedrichstraße have elevators....am well familiar with both places. Or, take a Regional Bahn train from Schönefeld to Friedrichstraße, not the S-Bahn, which is not direct. Can't help you with U-Bahn 6 to Naturkundemuseum, towards Tegel. Hope this helps a bit. If you could land at Tegel instead of at Schönefeld, it would be less cumbersome...probably too late for that.
Thanks about the construction around the Hauptbahnhof. Unfortunately the flights to Tegel were too expensive...