It probably doesn't matter, but I'd probably play Serenity first (if you can stick through my rough first chapter). You can really play either game first because if you play Last Human first, you can always go play Serenity to learn about Rin and where she came from.
Discovered serenity from this game, Kind of wish I had started with Serenity first as the crossovers make more sense. Though I am liking the upgrades being staggered between the stories so they interlock more.
