Is it better to insure for market value or agreed value?

Is it better to insure for market value or agreed value for car insurance in Australia, agreed value better for certainty and newer cars under 5 years, market value cheaper 10% to 20% less premium for older cars over 5 years. Agreed value means insurer pays fixed amount $20K agreed at policy start if total loss, regardless of depreciation, giving certainty and covering loan $18K, costing $900 per year for $20K car versus market value $800 per year saving $100, but payout $20K versus market $16K to $18K based on RedBook at time of loss. For newer car under 5 years bought $30K, market value $25K after 1 year, agreed value $30K better as you get $5K more to replace and cover loan $25K, worth extra $100 premium. For older car over 7 years $10K, market value $8K, agreed value $10K, difference $2K, premium difference $50, market value okay as depreciation slow and cheaper. So better to insure for agreed value for newer cars under 5 years, financed cars, and if you want certainty $20K payout, costing 10% more $100 per year but worth $2K to $5K extra payout. Better to insure for market value for older cars over 5 to 7 years, low value $10K, if you want cheaper premium $50 to $100 saving and accept lower payout $8K versus $10K.