The Siren Visual Blu-ray has the best picture quality of all English releases, but just like the Sentai Blu-ray it's unfortunately 1080i 29.97fps instead of 1080p 23.976fps. Some minor compression artifacts creep up from time to time.I own the Siren release from Australia. I think it's fine, although I haven't rewatched the whole series and am not a videophile.
I'm hoping Sentai will revisit Penguindrum someday, because their compression is so much better these days.