What REALLY is the difference? Chansey has overall more bulk and can take physical hits a little better. They have the same movepool as far as I know so they're using the same things. The only thing Blissey has is a little more base HP and Leftovers. I just know Blissey was OU at one point whereas Chansey never was, were people just sticking to Blissey because they were used to Gens 2-4 without Evo Stone?