I loved KMHM.

I avoided Hyde, Jekyll, Me for a long time because I adore Hyun Bin and most of the reviews I read said HJM was bad. I finally gave it a shot the other day and...I couldn't make it past the third episode. The story was a mess.
I have not watched both yet. Waiting for vacation to happen and watch both in marathon - few days only. I will start with Hyde, Jekyll, Me first as a lot of people said it was boring. The reason for it, they compared it with Kill Me, Heal Me so that is why reviews on HJM are so low. But I like Hyun Bin and Ji Sung and I don't like to hurt ether of them for a simple reason that one drama is with the better script written then another. Both actors are phenomenal and I want to be fair to both so I am starting with less popular drama 1st before I hit KMHM. 
Kill me, Heal Me. I couldn't get past the first episode of Hyde, Jeykll, Me. I took the gorilla as a sign this drama wasn't going down a good road. Based on recaps it seems like I made the right decision. 
I haven't seen HJM but i think it would take alot to knock off as the better of the two.

Ji Sung is amazing .. to portray 7 different personalities must be so difficult but he does a brilliant job. 

KMHM is in my top 5 best K-Dramas
I completed them both one after the other *accidentally, though as I didn't read the resume of H,J,M xD* and Kill Me Heal Me is definitely better in my opinion but I liked Hyde, Jekyll, Me as well. 
Kill me, Heal me ALL DAY!! I LOV THE ACTING THE OST AND THE PLOT LINE!! YOON AH!! I never skipped a second coz it was amazing!
I did try to watch Hyde Jekyll but I couldn't make past episode 12 (I had to drop it) I just couldn't bear to finish it.....
Definitely Kill Me, Heal Me!
I completed both Kill Me, Heal Me and Hyde, Jekyll, Me  , so believe me when i say thaat Kill Me, Heal Me is faaaar away from HJM !
i definitely advice you to watch kill me hill me !! and you wll fall in love with the story and the chemistry between Ji Sung && Hwang Jung Eum is perfect  <3
Kill Me, Heal Me is waaaay better. Vastly more fun to watch too.