Spiritomb barely dents Hypno, not to mention Hypno has reflect and can nasty pass if you're relying on that. I find Hypno to be a lot scarier because unlike Chansey, he can actually deter physical threats from switching in.
Well, I've actually been using Counter Chansey lately (yes, I know it's gimmicky, and so 2003), so I don't actually mind some physical threats switching in. Not as many OHKO Chansey as I thought, especially when I predict well and Softboiled on the switch. LO Dugtrio, for example, typically deals about 500 HP to my Chansey, so Counter deals 1,000 HP damage back (not that it needs to)! I've found Dugtrio is a very common switch-in to Chansey, so I love it when my Chansey OHKOs it! The difficult ones are the ones with Swords Dance, but I usually play safe, and use Thunderwave, although it does seem like such a loss if they attack you, and you know you would've KOed them if you'd've used Counter instead. But obviously, it's probably worse if you use Counter, and they Swords Dance, as most things OHKO Chansey after Swords Dance, and then the rest of you team has a +2 Pokemon to deal with.
And I would hardly say that Spiritomb "barely dents" Hypno. I mean, it learns Sucker Punch, Pursuit and Shadow Sneak. The Spiritomb that I'm using at the moment has 2 of those moves.
Hypno is better in some ways though. It can actually use special attacks, it gets Reflect, it gets Hypnosis which is much better than Sing, you can carry a physical move if needed (which Chansey certainly cannot do), you get Baton Pass, Nasty Plot and Belly Drum, and Hypno can actually effectively use Calm Mind.
I prefer Chansey in general, but Hypno does have its uses.