I've watched the kdrama version and i liked it. It only had 2 episodes, each episode is 1 hour 10 minutes. The story was fast paced and nicely developed, but i feel like the ending a bit rushed.

This version was okay. Maybe because there's more episodes (24), it get stretch out and focused too much on the school part. The added side stories making the plot's focus slowly faded. Making it less of a time travel and more of youth drama.
I do like the school part, it was funny and nostalgic, but it should've only focused on the main character's story. And even with more episodes, the last few episodes still felt rushed.

Overall, it was enjoyable
I skip lots of the filler scenes, but i still enjoy most of the drama. Also the songs used were really good too, even though they overplayed it a bit.
If you're watching this don't hope much for the time travel plot, just enjoy it as it goes.