Jan is definatly better thief than nalia (as someone else already stated she has just enough thief skills to be dangerous to the party. heh you THINK you are safe from traps since she is searching for them...). Imoen is passable.
Jan has great adventuring skills, he just lacks the combat thief type of stuff (he isn't really fantastic at sending in for backstabs etc).
Besides Jan is just too much fun not to have in your group
