Anyone who withdraws cash at an ATM abroad often has the choice of whether the debit should be made in the local currency or in their own currency. Which option is the cheapest?
Local currency is usually cheaper
To cut a long story short: In general, it is usefull to make the debit in the local currency. This allows you to leave the conversion rate to your own bank, credit institution or credit card issuer. In general, you are on the safe side because your local banks use a valid daily exchange rate.
If, on the other hand, you choose the option that the foreign ATM operator converts the amount into your home currency (for example Euro or Dollar), he can determine his own exchange rate. This is usually much worse than the actual daily exchange rate of your house bank or card issuer.
This also applies if you are offered a conversion directly at the card reader in shops.
The tricks of foreign ATM operators
Of course the foreign ATM operator tries to lure you to the offer to convert into your currency. For example, you will be offered no commission fees. This may be true, and you save yourself a flat fee of some bucks for the payment, but even with this commission fee you usually get away cheaper than with the up to 15% more expensive exchange rate.
Beyond that there are still further tricks used by the cash machine operators. In Croatia, for example, I found an ATM where I had to confirm twice that I really want to choose the local currency. The trick was that the button for the national currency was located on the left of the first screen, then on the right of the second prompt screen. If you don't pay attention here, you can easily switch to the conversion to your home currency again in the second query by fault. Asshole-ATMs!
"Tourist ATMs" should be avoided
Another tip on my part is to avoid the typical "tourist trap ATMs". So don't use any ATMs at kiosks, sights, train stations, airports and so on, but use the ATMs at branches of big and well-known banks. For example, I don't use "Euronet" ATMs myself. :)