The average charge for currency exchange (loading charge) is 2.75% but you also have to weigh up interest rate and interest free days to truly compare cards. Cahoot offer about the lowest I have seen - 1.25% and also currently have a low interest rate (8%), however no special intriductory rate.
A good bet for a holiday like WDW is to get a card offering 0% interest for a period. Good ones just now are Capital One - 0% for 6 months (loading rate of 2.75%) and this will work out cheaper than cards with lower loading charges but which have interest to charge (provided you do pay it back by the end of the 6 months).
Egg are also a good bet currently offering a 0% introductory rate - I think until October and they have a slightly lower loading rate at 2%.
These cards work well if you do clear your balance within the introductory period but if you don't you may end up eventually paying higher interest than some other cards, so you need to work out if you will clear your bill in time or if another card with a low general interest rate is better.
Jackie