I watched She Was Pretty first. I'm now currently viewing Kill Me, Heal Me.

I personally think that Park Seo-joon and Hwang Jung-eum have better chemistry as siblings than they did as a couple.  I really like their interactions in KMHM...I think that might be because I like his character more in this show than I did she was pretty.

But..I can't help but wonder why his character has all that information on Do Hyun....

OH I loved both k-dramas so much. They were one of my earlier first dramas I watched and I'm pretty sure they were one after the other as well. I, too, agree that I like seeing Park Seojoon and Hwang Jungeum as siblings rather than a couple. However, I still enjoyed She Was Pretty a lot. I watched these nearly two years ago and now I feel like rewatching them haha ;P
She was pretty was one of my first dramas also... Siwon's character Shin-hyuk is still my favorite kdrama character of all time.
I've NEVER had second lead syndrome of the scale I did with him. 
I enjoyed She Was Pretty also, I just wish the second lead had gotten the girl on this one :) 

Also...Kill Me, Heal Me is a phenomenal drama and I'm so glad I watched it.  Ji Sung is an amazing actor and can portray so many different character types and do it so well.
Really makes me think if Hwang Jung Eum will ever do a normal girl drama lol.

All her romcoms, her characters are weird (or annoying)
Kill me, Heal me is my favourite drama so I vote for it. She was pretty was also quite enjoyable tho but I didn't really feel the chemistry between Seo-Joon and Hwang Jung Eum which might be because of the second-lead syndrome
I love both dramas but if I have to pick one, it's definitely KMHM. The plot twist is just amazing