For the m2u, you need to go to atm machine and activate it then u can use it for online transaction already. In term of point then it was Maybankard Machester United Visa Debit more better but if in term of exchange rate then it will be Mastercard more better as it offer the lower rate....   But recently I think mastercard and visa is the same..... Please correct me if I'm wrong

Have you activated the m2u online banking?

Yes. It's a bit different. You can check out their website for the differences 
I have never used my ATM card to withdraw cash in South Korea, thus i can't advise you on this. But do note that there is a 0.5% admin fee for all overseas transactions and this include overseas ATM cash withdrawals + RM10 fee for every withdrawal.

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 
I still remember that there was another person using this Maybank debit Mastercard saying that this card cannot be used at several online sites. Google Wallet could be one.