Want advice between prepaid sim card or phone rental for 2 wks trip

I don't know what phones cost to rent for a short stay in Korea. I don't believe they would be expensive, have seen the rental shops at Incheon airport in Seoul. I do know that when using a sim card the cost of calling locally is 500 won per minute. Text messages am not sure but everybody in Korea who use smart phones use the app "Kakao Talk" which is free as it's via the internet. Wifi IS literally everywhere in Korea.

Many Thanks so much my friend, Do you know if my Samsung galaxy S2 cell can be compatible with 850/900/1800/1900 mhz system, and compatible with 3G system of 850/2100 mhz, can I just buy the sim card there and which mobile operator has the cheapest cost of calling?

-I download Kakao from what you mention, does it mean I can chat with my friend with wifi without if my smartphone can't make a call there? Kakao said it offers the FREE phone call with the people in the network, do you think it's true?

First of all I don't have a smart phone so I can't answer your initial question. Thats way over my head sorry.

As for the latter, all I can tell you is that I have an American friend here who has her phone that she had in the US. It's a galaxy, don't ask which model, couldn't tell but she uses Kakao talk all the time and her one is the one she bought in the US prior to coming to Korea. She still pays for her contract back in the US and uses it here. As am not a big cell phone user I can't really answer those questions with clarity.
Post a question on koreabridge.net, there are many expats there that should be better informed on this technology than I am.

Cheers