IMO Blissey is better than Chansey for one paticular reason: without Leftovers you severely compromise the ability to be a status absorber. To my surprise awhile again, taking a burn is bad news for Chansey, especially in situations where you are pressured to stay in. Burn inflicts 25% to Chansey after two turns as opposed to 12% for Blissey! Even something as mundane as paralysis causes trouble since Chansey is completely reliant on a turn in order to heal; while the small amount offered by Leftovers is enough to buy some time. Yes, you can switch out of the burn, but when you are under pressure to not to switch out (for exapmle aginsy Heatran) sometimes that oppurtunity isn't avalible.