Car supermarkets vs dealerships

I've searched the differences between buying from a supermarket vs dealerships and the general consensus is that dealerships will give you a better warranty and after sales servicing. I just wanted more opinions based on experiences of buying from one or both. Is the warranty really that much better that it's worth paying more money at a dealership? Are supermarkets more likely to scam/hide faults with the car such as previous accidents?

I'm looking to buy the car pretty soon and would appreciate any advice?