I agree with Mad.
Baron Wittard has some extremely difficult puzzles -- if your brain hasn't been fried by Rhem 3 and you're up for a pummelling, then go for it.
Black Mirror II has a wonderful, involving storyline, but the puzzles are much easier (there's also a puzzle skip feature).
Baron Wittard is solitary first person, whereas BM2 has lots of dialogue and character interaction.