"Constantin English Variant of Constantine: Steady, stable. Male"
Baby Names Starting with C, page 9
So according to this page, there is no real difference.
The site also says
"Constance Latin To be knowledgeable. Female"
"Constant English Variant of Constantine: Steady, stable. Male"
So Constance and Constant have two different meanings, but Constant means the same thing as Constantin and Constantine.