There can be a problem using cards in France. Their credit and debit cards have security strips rather than magnetic stripes so French ATMs may not be able to read yours. With cards and travellers' cheques, it pays to shop around - different banks have surprisingly different charges for the same service. Cash withdrawals on credit cards can cost between one and two percent of the amount withdrawn with varying minimum charges. When you buy travellers' cheques you normally pay a percentage of their value in commission charges with a set minimum.
Some banks charge no commission for travellers' cheques ordered on line.
If you buy them in the local currency you don't normally pay a commission when cashing them in, but if you take sterling travellers' cheques you do - and often get a poor exchange rate too. The big advantage is that if they're lost or stolen replacements are normally provided very quickly. Finally, remember to keep separate records of card and cheque numbers in a safe place.