Credit Card is definitely the way to go. You get the best exchange rates this way but be aware you will be charged a fee each time you withdraw money at an ATM.
If you have Card Protection (costs about €20 annually if I remember correctly), this would cover you in the unlikely event you lose your card e.g. fraudulant purchases and advance you money to tie you over to the end of your holiday if necesssary.
One other thing, in certain parts of the USA particularly in California, it is customary to ask to see ID when using your credit card.