Compressing ~125MB worth of data for every in-game turn makes the end-turn lag long. But for manual saves, numerous 125MB files can be unaffordable.
Hence, the suggestion to treat the two differently. If one single toggle is desired, I would suggest always making auto-saves and exit-save uncompressed.
Related, currently the game would not read compressed game saves if the setting is to not save into compressed files. It'd be nice if reading is compatible either way.
Adding to that, currently the compressed saves don't have extension in the filename aside from .gz -- judging by the file size saves are always json files? That would make manual re-name impossible pending on what setting the player change from since the player would have no knowledge of the extension the game would read from with the no compression toggle. Making reading compatible for both uncompressed and compressed saves would solve this issue as well.