I opted for back to invoice but it really depends on your circumstances and what deposit you've paid, how much finance you'll owe etc.. Basically I'll try and summarise the differences:
Back to invoice:You paid £15k for the carYou owe £11k on financeInsurance only give you £10k.
Gap will make up the extra £5k shortfall allowing you to pay off your finance and essentially start again with a £4k deposit.
Vehicle Replacement:Same situation as before;You paid £15k for the carYou owe £11k on financeInsurance only give you £10k.
The difference here is due to price increases a new Adam now costs £16.5k so the gap insurance will make up the £6.5k difference allowing you to get back into a car again without losing out and having to get something cheaper.
You might also be in a situation where you owe more on finance that what you originally paid for the car, in that case back to invoice would leave you short. Vehicle replacement would cover the cost of the shortfall.
Hopefully that helps / makes sense. I don't work for an insurance company so the above is my understanding of how it works