Is it better to bring Dollars or Euros? Is it possible to pay with creditcards (mastercard or visa) in most places? Like hotels and such. What is best to do, get cash or pay with creditcard?
Bring whatever suits you best. If you live in the Euro area there is no point in converting Euros into Dollars to convert them later into Won. You will loose twice from the exchange. Euros can as easily be exchanged into Wons as Dollars can be. Buying Won in Europe can be cumbersome and is not worth the effort.
Credit cards (MasterCard and Visa) are also widely accepted. You can even use them to withdraw money from automated teller machines (ATM).
I usually bring a bunch of Euros, and exchange them progressively into Won. And I use my credit card intensively. This way, I end up using all my Wons and go home with some Euros in my pocket. But there are alternative valuable strategies.