Well, you've partly answered your question already.
There are two key components to foreign card transactions:
1) Foreign currency transaction fee (it is usually fixed, however some banks may not charge this, but rather have higher spread - you need to check with your bank)
2) Exchange rates
Also, banks may have all sorts of extra charges such as an ATM withdrawal fee for example.
The FC transaction fee is charged when, say, you make a JPY purchase with USD or EUR. This transaction will also involve some loss on x-rate (to convert USD or EUR to JPY obviously). But before you buy JPY, you will have to buy USD or EUR with PLN.
Because you said you are more likely to travel to Europe it is then rational to use a EUR card as it won't involve a FC transaction fee. You will still have to convert PLN to EUR, or if you have income in EUR or USD you have nothing to worry about then.
In practice there is no such thing as spread-free exchage rate. Every institution that sets xrates has to cover its expenses - this includes both governments and banks. Assuming your relatives do not gift you the currency, they will have to exchange PLN you send them back to USD or EUR which would in turn involve a financial intermediary.
One may also consider calculating the exact benefit of using either USD or EUR card abroad, say, Japan. However, usually the difference is insignificant for day-to-day expenses because the market mechanism ensures that there is no free profit opportunity available. If there happens to be a gap and an opportunity for free profit (i mean converting currency A to currency B then to currency C and back to currency A, pay for all the transaction fees and still make some profit) such an opportunity will be quickly closed off by tradebots and FC market will be in equilibrium again instantly. If this argument is not strong enough for some reason, consider another one - assuming you are getting FC card for long-term then constant x-rate fluctuations would make it almost impossible to predict which currency would have more purchasing power in the future. You could, ofcourse, hedge but this is another story and barely related to consumer cards.
Long story short, the first scenario includes double conversion plus a FC transaction fee, whereas in the second case scenario you only convert once and spend without any FC transaction fee. It could be that you may happen to travel to Japan with EUR card, in this case you would be subject to double conversion and a transaction fee as in the first scenario. Therefore it is rational to use EUR card, as you if you were using USD card you would be sebject to double conversion within the EU. Alternatively, if you are more likely to travel to US you would be better-off with a dollar card.
To summarise - the more conversions you make the more you lose. And if you somehow managed to calculate that today you are better-off with a USD card - everything could change tomorrow.
