Really interesting!
I feel the length of both game is good, seeing how the story progressed so far. My first game was Serenity, but i'm more attached to Last Human story wise, so i'm up for any extension of its story!
Talking about a 3rd game that is so far away is strange, i may change my mind easily over time :D i think both story are solid backbone but the 2nd one is more unique since it seems focused on one main girl, opposed to the multiple/harem girls of most AVN. (somthing like Melody?)
Personally i'm more into the first story, but at the end you must work on something you would like to do for a long time!
Thanks for the update!