Hi, It depends where you are travelling and how you spend, either cash withdrawals or purchases.
Try to get a card with the same curency as the country you are in, fairfx $, euro and sterling cards are good value and give good rates of exchange when loading.
Often larger and chainstores will give good rates of exchange for dollars in the Americas as the proprietors want dollars!!! sometimes up to 15% more than the banks in mexico for example.
In Europe i would guess a euro card would be best value, although i don't have much experience there just USA and Central America
Have a good trip!!!