Mepher's answer is comprehensive, but if you want to settle on one I would recommend "differ from". The thing to remember is that we are talking about the opposite of being similar to something. Since "to" and "from" are opposites, it's logical that the opposite of "(be) similar to" is "differ(ent) from".
I know that English isn't always logical but in this case it conforms nicely. As an aside, a real pet hate of mine is "different than" - it makes me wince every time I see or hear it!