Eviolite Chansey is better in most situations. It has better bulk overall, specifically in Defence where Blissey is extremely lacking. This stops Chansey from being worn down by physical attackers too quickly. The only thing that really gives Blissey a reason to be used is Leftovers, but that isn't a good enough reason to drop the extra bulk. Blissey also has a better Special Attack stat that is worth keeping in mind, although sets the use this are rare and force you to leave out one of Blissey's other moves. Seismic Toss is better for dealing damage.
Both of them should use max HP and Defence, as this gives them the best chance against physical attackers as possible. Their Special Defence is already good enough due to its base stat and the huge HP, so EVs are better left to Defence. Bold is a good nature for both of them, as it lower the useless Attack stat and improves Defence slightly.