Hey mgeorge,
I can't comment on Tunguska as it's still on my "to be played list" but I'm currently playing Nibiru myself and I'm really enjoying it. I think the story is good and the puzzles make sense for the most part (I've only got stuck once and had to refer to UHS for a hint.
) The main thing is Nibiru has kept my interest and attention so far and that's saying something for me.
Thanks,
Tony.