Which Chansey?

Hard to tell without knowledge of the CP, but its pretty marginal between the two. Go with the second, attack stat matters most on Blissey.

People will say attack is most important, and it is for CP. But the more I think about Blissey, the more I conclude that maximum tankiness is the goal. So Def IVs may be most important since Stam is already so high.

Does Blissey typically kill mons or just time them out?

Edit: Boo whoever downvoted without a response.

This will make absolutely no difference whatsoever.

But since you have to choose, I would chose the 82% IV one. Blissey is all about being a defensive tank and making opponents time out, so better defense is desirable (in practice, 2 extra defense points is more impactful than 1 extra attack point). The 8 CP lower shouldn't make much of a difference in gym placement.

But either are strong and would make good defenders.

I would go for thw 15 attack first, as Blissey stats are
129 Att
229 Def
But someday you'll want both evolved either way (hopefully with good moveset, and then power 'em).