They cover different events. Zero depreciation applies to repair claims: the insurer pays the full cost of replaced parts instead of deducting depreciation (50% on plastic and rubber parts, an age-based share on metal parts). Return to invoice applies only if the car is stolen or written off: instead of the IDV, the insurer pays up to the invoice price, often with road tax and registration. Zero dep is used often; RTI rarely, but it protects a large sum. Many buyers of new cars take both in the first years.
See on-road prices, with registration, road tax and insurance included