I decided to try out berlinlinienbus with their online promotion Hamburg-Berlin for 7 euros.
The trip is about 3,5 hours ( only 1 hour 40 min by train) but the sits are comfy and buses are not super full, so one can take 2 sits easily. A ticket allows 2 bags 25kg each. Busses run between Hamburg ZOB and Berlin Ostbahnhof/Berlin ZOB. No internet onboard (for this price whatever), drinks, coffee, and snacks are available onboard.
The station is another option to get food and coffee before the trip. The location is quite far from everything, but sbahn station is 5 min walk only. Take ringbahn to speed up your commute. read more