Tsui no Sora can only be described, as perfection.
The author, Sca-JI, peaked with the creation of this grand masterpiece. Subahibi and Sakura no Uta look like complete trash in comparison to the mastery Tsui no Sora has achieved.
In fact, Subahibi isn't even needed as a remake, when all it is, is a downgrade.
The OVA is an excellently told story with great philosophical themes somehow fleshed out immensely within its 24 minute run. Where as Subahibi drags on and on with Ayana spouting nonsense everytime she appears on screen, Tsui no Sora communicates everything Subahibi fumbles at effectively in 24 mins, as opposed to reading an 80 hour VN to hear the same shit told to you but in a worse way.
Mamiya Takuji's godly being is on full display in this OVA. Who needs something as stupid as the character development, or motivation, or build up for Takuji like in Subahibi, we don't need any of that shit, it's a waste of time. Tsui no Sora doesn't waste our time with snoozefests like that and instead gets straight to the point.
Overall, Tsui no Sora is an excellent, thought provoking metacommentary on whether Australia was right to ban hentai. If you're considering reading Subahibi, don't waste your time and watch this instead.