Debit or credit: what actually changes
They look the same, they tap the same, and one of them is dramatically better to have stolen.
Both cards look identical at the terminal. Underneath, one spends your money and one spends the bank's.
Whose money is moving
A debit card is a remote control for your bank account. Tap it and your own balance falls.
A credit card is a small loan agreement that happens to be shaped like a card. Tap it and the bank pays the shop and adds a line to what you owe. Nobody has touched your account.
Everything else follows from that one sentence.
The asymmetry nobody mentions at the counter
Dispute a fraudulent credit card charge and you are arguing about a number on a statement. The bank is out of pocket, not you, and you have not yet paid. You can be patient because nothing has left your account.
Dispute a fraudulent debit card charge and the money is already gone from the account you pay rent from. You will very likely get it back, and the timing is the problem, not the outcome. Provisional credit takes days, and rent does not wait for provisional credit.
Same fraud, same protections in outline, completely different fortnight.
This is why the practical advice is duller than it sounds: use credit for anything online, anything abroad, anything with a deposit, and anything you would be upset to lose access to money over.
The economics behind the shop's face
Cards carry different interchange. Credit generally costs the merchant more than debit, and premium rewards credit more still. In some markets debit interchange is capped by regulation and credit is not, which is why the two behave so differently commercially in the same shop.
This is the whole reason for the small sign near the till, the minimum spend on cards, the surcharge where surcharging is legal. The shopkeeper is not being difficult. They are looking at two numbers you cannot see.
And it explains India's peculiarity: with UPI and RuPay debit at zero MDR, a shopkeeper has an active preference about which rectangle you pull out, and it is usually neither.
The third thing in your wallet
A prepaid card is neither. It spends a balance you loaded, held by a company that is often not a bank, and the protections are different again. Worth knowing which of the three you are actually holding, because the plastic will not tell you.
Paying from your own wallet, or asking the shop to put it on a tab someone else settles at the end of the month. Both buy the sandwich. Only one of them leaves your wallet lighter before you have decided the sandwich was any good.
Where you meet it
Every checkout asking "debit or credit", every hotel that wants a credit card specifically, and every card minimum sign taped beside a till.