I can't go very in depth jonny, but Issey just seems better crafted to me. I've worn both and AdG just disappear on me; Issey does not.
Part of me like the original. I believe both make use of calone, which has an aquatic vibe, and Issey was first. Personally, I think AdG was a ripoff of Issey. Through marketing muscle and money, AdG passed Issey in sales.