OP states they want the most efficient use of time and in this case, flying is it - folks more on a trip around Europe and not on a time squeeze may enjoy the train ride.

Train was the best way for me. And it was not so slow(5 hours from Frankfurt Oder). Flight is not much faster as you have to invest 3-4 more hours for preparing and airport transfer or public transport.

I'll be doing the opposite in the fall - Warsaw to Berlin. I prefer the train since it gives me a chance to relax and recharge my batteries after a rather hectic journey. I enjoy flying, it's the airports that stress me out. I find Man's in Seat 61 website useful and informative.