Speaker for the Dead made me think even more. But I found it interesting that one of the concepts directly opposes Star Trek's Prime Directive. I will likely be thinking about that off and on for quite some time to come.
But Xenocide I am having some trouble with. For one thing, I feel like I am missing a whole volume of the story. For another, I am not sure what story is being told. It's just kind of ...odd. All of it, really. There's no part I can point to and say 'well, that part's not odd'. If I were reading, rather than listen, I think I would have abandoned it, I am failing so utterly to click with it.
Edit: Spoilers in the comments. Not that that should matter, as the book are so old.