BOI/Mastercard in the USA

Hi All,
I am going on holidays to the USA next week and I am wondering should I buy dollars and bring them as cash or credit my Mastercard and use that while I am there? For convenience I'd prefer to load my credit card, however I think that my euro may go further by buying some dollars? Are there different exchange rates for buying dollars in the bank and for credit card transactions on any given day?

Thanks

When I visited the US I used both cash and my BOI M/C.

"When I visited the US I used both cash and my BOI M/C"
How is this post of any help to buckfast Smashbox.

If you preload your mastercard and it is stolen or skimmed you are fully liable for however much you are in credit. Exchange rates can and do change daily so it is nearly impossible to predict whether you are better off with cash or not.