Snorlax gym defense: ZH/BS vs ZH/HS
ZH/BS does more neutral damage, and BS is spammed more often and harder to dodge, but it does much less damage to Tyranitar than HS.
Answers
Snorlax deals more damage to the attacker than Chansey. Sims have Machamp finishing Chansey with more HP left dodging nothing than Snorlax with realistic dodging specials. I agree that Snorlax is weaker because time to defeat is the most important metric to me and Snorlax loses to Chansey there, but against an attacker low on revives you'd rather have Snorlax.
Dragonite wins ZH/DG Blissey too if you're decent at dodging. Kind of a moot point because you'd rather use Machamp to defeat any of them anyway due to how much faster it is.
Speaking as someone who only ever really uses Machamp/Hariyama to take Blisslax down, I'd much rather face HS than BS. The latter is a rare-ish legacy, but more importantly it's so unpredictable and can be a nightmare to dodge. Maybe I'm just far more used to facing a HS Snorlax than one with BS. Either way, if you've got a BS Snorlax, just keep it that way.
Edit: just for the record, BS = Body Slam. I know this reads funnier if BS were bs but it's not.
If the attacker is picking Tyranitar to counter your ZH/BS Snorlax they've kind of lost already because they're using more of their precious time than they would using fighting types.
ZH/BS is definitely better because BS spams so often and is harder to dodge. The only reason to have HS is that you can't get BS anymore.