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Using Chansey is going to be a lot riskier than Snorlax. Chansey may have a lot of HP and Sp.Def, but its Def is basically non-existence. It's got pretty good Sp.Att though. Meanwhile, Snorlax has sizeable HP, good defenses on both sides and pretty good Attack. I'd say Snorlax is a lot better unless you really like Chansey.
An easier way to determine which one to use is that Snorlax is fully evolved. Chansey is not. Snorlax, if memory serves, is also stronger from his Gen 1 days while Chansey got nerfed believe it or not.