My opinion only:
Specialist credit card is the best for overseas spending:
A specialist credit card with "no sterling exchange rate" (i.e. Halifax Clarity Mastercard plus many now unavailable credit cards) is, and always had been the cheapest and most popular way to spend abroad,
Pros - if you have a good credit record and repay in full each month, exchange rate based on the day you use it, plus "Section 75 protection".
Cons, if you've not a good credit history and you can't pay off in full each month.
This type of card is best for car/bike rentals, hotel/restaurant bills but NOT usually for use in ATM's.
Unfortunately not all tavernas/bars (but most do) in Agios (San) Stefanos accept cards, so cash is essential too, bought in UK before travel invariably best.
Normal credit cards usually impose foreign usage/cash withdrawal/credit card interest and flat fee, not wise to use.
Pre Paid Cards
You get the exchange rate when you loaded it, so you could win or lose on the day you spend, you pay your money and take your chance.
Pros - you can't overspend (so good for teenagers), no credit check, widely accepted, easily blocked if lost (but can incur telephone fees), lower or no ATM transaction fees (on some).
Cons - Many, but NOT ALL prepaid cards come with, typically, application fees (£2-£10), renewal fees (£2-£7) top up fee, monthly usage fee (£2-£5) ATM/Transaction fee (on some 3% or £2), Inactivity fee (if not used for 3 months), Redemption fees (getting unused money back, average £10), emergency replacement card (Up to £35 for phoning customer helpline), cross border fees within EU (2.75%), no Section 75 protection.
Note - these are only typical charges, different providers will vary.
Do the benefits of this card outweigh the costs? Only you can decide.
For me, I'll stick to the various "specialist no commission credit cards" that I have used for years. I've even won a bet on the benefits of this type of card from a notorious forum member, no names mentioned ------ unless he replies!
My opinion only, make up your own minds.
This may help:
http://www.moneysavingexpert.com/travel/cheap-travel-money