Not sure where the Marriot is located, but the most convenient and affordable way of leaving TXL to get into the centre of Berlin is via the X9 bus. It departs from the bus stand outside the terminal and takes 15 minutes to get to the Zoo train station, which is a central transportation hub, with buses and underground trains that will take you to any part of Berlin. The cost of a one way ticket is euro 2.10. And the same ticket allows you to connect on on any mode of public transport within the city. Check the website bvg.de (also available in english) for a comprehensive look at public transport in Berlin. Enjoy the trip.