I think the main difference is that Chuck Taylors come in a high top version. I don't think the Jack Purcells come in a true high top.
I also think the construction is a little different. I think the Jacks have more support and are a bit more stylized than the Chucks. I think Chucks (whether high top or low) are more classic looking.
I could be totally wrong, but that is the difference that I personaly notice.