Both videos are different music videos of the same song. I prefer the first one as it tells a sad story of the girl not wanting her guy to know about her illness and get hurt due to it. His friends played along but ended up telling him. However he did not manage to say his last goodbye.

떠나가
ye finally i realize that i’m nuttin’ without you
i was so wrong forgive me
ah ah ah ah –
파도처럼 부숴진 내 맘
바람처럼 흔들리는 내 맘
연기처럼 사라진 내 사랑
문신처럼 지워지지가 않아
한숨만 땅이 꺼지라 쉬죠
내 가슴속엔 먼지만 쌓이죠 (say good bye)
네가 없인 단 하루도 못 살것만 같았던 나
생각과는 다르게도 그럭저럭 혼자 잘 살아
보고싶다고 불러봐도 넌 아무 대답없잖아
헛된 기대 걸어봐도 이젠 소용없잖아
네 옆에 있는 그 사람이 뭔지 혹시 널 울리진 않는지
그대 내가 보이긴 하는지 벌써 싹 다 잊었는지
걱정돼 다가가기조차 말을 걸 수 조차 없어 애태우고
나 홀로 긴 밤을 지새우죠 수백번 지워내죠
돌아보지말고 떠나가라 또 나를 찾지말고 살아가라
너를 사랑했기에 후회없기에 좋았던 기억만 가져가라
그럭저럭 참아볼만해 그럭저럭 견뎌낼만해
넌 그럴수록 행복해야돼 하루하루 무뎌져가네
oh girl I cry cry yo my all (say good bye)
길을 걷다 너와 나 우리 마주친다해도
못 본척 하고서 그대로 가던길 가줘
자꾸만 옛 생각이 떠오르면 아마도
나도 몰래 그댈 찾아갈지도 몰라
넌 늘 그 사람과 행복하게 넌 늘 내가 다른 맘 안 먹게
넌 늘 작은 미련도 안 남게끔 잘 지내줘 나 보란듯이
넌 늘 저 하늘같이 하얗게 뜬 구름과도 같이 새파랗게
넌 늘 그렇게 웃어줘 아무 일 없듯이
돌아보지말고 떠나가라 또 나를 찾지말고 살아가라
너를 사랑했기에 후회없기에 좋았던 기억만 가져가라
그럭저럭 참아볼만해 그럭저럭 견뎌낼만해
넌 그럴수록 행복해야돼 하루하루 무뎌져가네
나를 떠나서 맘 편해지길 (나를 잊고서 살아가줘)
그 눈물은 다 마를테니 ye (하루하루 지나면)
차라리 만나지 않았더라면 덜 아플텐데 um
영원히 함께하자던 그 약속 이젠
추억에 묻어두길 바래 baby 널 위해 기도해
돌아보지말고 떠나가라 또 나를 찾지말고 살아가라
너를 사랑했기에 후회없기에 좋았던 기억만 가져가라
그럭저럭 참아볼만해 그럭저럭 견뎌낼만해
넌 그럴수록 행복해야돼 하루하루 무뎌져가네
oh girl i cry cry yo my all say good bye bye
oh my love don’t lie lie yo my heart (say good bye)

Leave
Yeah, Finally I realize that I am nothing without you
I was so wrong, forgive me
Ah ah ah ah~
[Verse 1; GD + TOP]
My broken heart like a wave
My shaken heart like a wind.
My heart vanished like smoke
It can’t be removed like a tattoo
I sigh deeply as if a ground is going to cave in
Only dusts are piled up in my mind
(say goodbye)
[GD Rap]
Yeah, I thought I wouldn’t be able to live even one day without you
But somehow I managed to live on (longer) than I thought
You don’t answer anything as I cry out “I miss you”
I hope for a vain expectation but now it’s useless
[TOP Rap]
What is it about that person next to you, did he make you cry?
Dear can you even see me, did you forget completely?
I am worried, I feel anxiety because I can’t get close nor try to talk to you
I spend long nights by myself, erasing my thoughts a thousand times
[Chorus]
Don’t look back and leave
Don’t find me again and live (on)
Because I have no regrets from loving you, take only the good memories
I can bear it in some way
I can stand in some way
You should be happy if you are like this
I become dull day by day (eh eh eh eh)
Oh girl I cry, cry
You’re my all, say goodbye…
[Verse 2; SR + DS]
If we pass by each other on the street
Act like you didn’t see me and go the way you were walking to
If you keep thinking about our past memories
I might go look for you secretly
[GD] Always be happy with him, (so) I won’t ever get a different mind
Even smallest regret won’t be left out ever
Please live well as if I should feel jealous
[TOP] You should always be like that bright sky, like that white cloud
Yes, you should always smile like that as if nothing happened
[Chorus]
Don’t look back and leave
Don’t find me again and live (on)
Because I have no regrets from loving you, take only the good memories
I can bear it in some way
I can stand in some way
You should be happy if you are like this
I become dull day by day (eh eh eh eh)
[Bridge; DS + SR]
I hope your heart feels relieved
Please forget about me and live (on)
Those tears will dry completely
As time passes by
[YB] It would’ve hurt less if we didn’t meet at all (mm)
Hope you will bury our promise of being together forever baby
I pray for you
[Chorus]2x
Don’t look back and leave
Don’t find me again and live (on)
Because I have no regrets from loving you, take only the good memories
I can bear it in some way
I can stand in some way
You should be happy if you are like this
I become dull day by day (eh eh eh eh)
Oh girl I cry, cry
You’re my all, say goodbye, bye
Oh my love don’t lie, lie
You’re my heart, say goodbye.

Lyrics from jpopasia.com