In general I would guess Psychic is better than Hyper Beam on defense. It's a beastly defensive move and really strong at damaging attackers.
Except Chansey is an exception.
With such low attack stats, Chansey isn't going to threaten anyone. The only threat Chansey brings is that you run out of time. So what's the counter to that? Don't dodge and just spam your attacks as well as possible. In which case you want Chansey to use Hyper Beam. Either they're going to waste time dodging it (which adds in to your strategy) or they'll take the hit which deals the most damage.