The problem with the new airport is that it is unclear when the fire protection systems can be certified. Apparently, quite a bit of rework is required. Air Berlin had hoped to use BER as a hub, but now they have to cram everything into TXL. The new airport should make connecting only easier. TXL will close when BER opens. I just would book a hotel with a cancelable rate so that you have the flexiblity to get something close to to BER in case it already is open by then.