You want the messaging app’s own paid features
The literal reading of the question and the cheapest branch.
Since the September consolidation, Meta’s paid per-app tiers are Meta One’s standalone rung at roughly $2.99 to $3.99 a month. So there is no “versus” to resolve: buying the WhatsApp plan is buying the cheapest Meta One option.
Whether to move up to Core instead is pure arithmetic on how many app plans you would hold, and it breaks even at two. That calculation is worked through in full on the Instagram Plus comparison, which applies identically here no point repeating it.