About G_P's Gunbuster, the last episode actually have 2 video tracks, one letterboxed (16:9) and one fullscreen (4:3). Also, G_P has more extras (though, I never really find the extras that important). Though I prefer the slight graininess in EtB's release though, fits these older anime more. Also, G_P has some onscreen texts typesetted that aren't present in EtB's, and karaoke (which I never find to be necessary). Personally, I'll get EtB and get the remaining extras from G_P just for the sake of completion.
Most probably this'll replace dp though.
On a side note, why is G_P's Gunbuster on B slot?