What is the difference between using a nice smoked malt vs. maybe using some liquid smoke for making Rauchbier? Can you detect the difference in complexity?
Is smoked fruit woods (apple, cherry) better than smoke from say hickory or mesquite?
I can't speak to smoked malt vs. liquid smoke.
But to answer your other question, there are big flavor differences between the types of wood you use, at least in cooking, so I would think it depends on the characteristics you want to impart in your beer. Here's a list I reference for the flavor profiles of different smokes (with a few I've added myself):
According to BeerAdvocate, Rauchbier uses Beechwood to smoke the malt.
I've never used liquid smoke before. Having said that, I think this calls for an experiment! For your first question (detecting difference in complexity between liquid smoke vs. smoked malt), try it! Make both, try them, and let me know :)
As an aside, when does one normally add liquid smoke?
As for your second question, about "better", that's a question of preference.