Thanks for the detailed info, OltonPete !
My impression from flying DUS compared to FRA on BE is that the DUS flight is busier and possibly more profitable (e.g. DUS normally more expensive on Fri/Sun esp short-notice, and still has a Sat flight), so my bet would be on DUS to get one of the first E175. I actually hoped they'd have a E190 already on occasion.
Agreed, trains from Hanover to Berlin are a little pricy (talk about a monopoly !), however similar to e.g. our Euston trains, one can get early-bird pre-booked 'Special Savings' fares for allocated trains, which start at 25 Euro one-way. From bahn.de, not Raileurope.