Maybank debit card is better if you are going to use it for overseas purchases as Maybank debit/credit card is known to give a very favourable exchange rate. Do note that there will be 0.5% admin fee for all transactions made in foreign currency. For Public Bank D2D VISA, the admin fee charged is 1%.

If you go for Maybank, i'd suggest you to specifically request for the Maybankard Manchester United VISA debit cardas this card gives you 1x Treatpoint for every RM2 charged to your card whereas the regular Maybankard Cherry VISA debit card gives you 1x Treatpoint for every RM3. Treatpoint is something like Bonuslink where you accumulate points to certain amount before you can redeem for a gift or shopping voucher.

Anyway, since you have a Maybank VISA debit card. I strongly suggest that you just use your Maybank VISA debit card to pay for anything at all places that accepts debit/credit card in Seoul. I've used my Maybank credit card many times during my last trip to Seoul just two months ago and the rate was KRW1,000 = RM2.73-RM2.74. I used the card to buy something online again and was charged in KRW just last month and the rate was KRW1,000 = RM2.73 for the first time and KRW1,000 = RM2.76 for another transaction done a few weeks later. As you can see, the final exchange rate is even lower than the rate offered by money changer in Malaysia which is around KRW1,000 = RM2.78-RM2.82 
Anytime