I use both websites, like a lot of other people here.
MAL is good for news, general infos, clubs and some discussion forums.
What I really dislike of MAL is how terrible the majority of discussions forums are, especially AD: users insulting each others, posts after posts of pointless "you are wrong, I'm right", topics that just make you wanna throw a chair at someone's head, and mods calming things down only after it has blown up badly (if and when they deem it worth it to show up at all).
I really enjoy using AL: the forums are now as lively as those on MAL, but I like how you can comment on your own and other people statuses, the whole thing is pretty user-friendly and generally everybody is well behaved.