Blackout has less, considerably less absorption coefficient due to the black out foil, however, the foil does create some diffusion. So it's not really a one is better vs the other, it depends on the room and placement.
How should I treat window behind listening chair?
My room is set up using Immedia mapping principal. I use a McIntosh 1700 receiver and JBL Alpha 1 speakers. A window is behind the ideal listening position. The sound is very good and there is no apparent vibration, but I would like to optimize the quality of the sound as much as possible. I'm afraid a drape will absorb too much sound.