Credit Card
Pay by credit card always - just make sure you pay it off on time!
You will get the bank rate for the transaction (ie: better than at a bureau de change etc.) which is better than the tourist rate.
Just check with your card issuer if they make a charge either flat-rate or percentage - if so may be worthwhile shopping around for a different card specifically for your trips.
And don't be using your credit card to withdraw cash from a machine anywhere - they start charging interest from that moment, even if you pay the bill in full each month. If you use a debit/Switch/Cirrus type card for ATM withdrawals then make it a decent amount each time.
Hope this helps.