A person who has at least 2 heads isn't normal.
A person who has 2 or more heads isn't normal.
Is there a difference between the usage of "at least [number]" and "[number] or more"?
They mean the same thing! In both instances, "at least [number]"/"[number] or more", you're saying that anything greater than or equal to the [number] is acceptable. Or, in this case, unacceptable.