They are written by the same person, so if you liked one you'll definitely like the other.
Honestly, I feel that the two lead characters are the same people in a different universe (somewhat like Cardcaptor Sakura/Tsubasa Chronicles).
Rando Musashi is a tough guy, an obnoxious karate star, the school troublemaker and... the most popular girl in school?! Rando was riding the bus home one day when a sudden crash resulted in a horrible wreck. He was immediately taken to intensive care under plastic surgeon extraordinaire Dr. Manabe, who treated Rando's maimed face. Yet unfortunately Dr. Manabe mistook a picture of Rando's school crush for a picture of how Rando looked before the accident. Rando awakens from a coma a full year later to find that he now has the face of Rina Kurimi, much to his dismay! He rushes out of the clinic in terror, only to run into Rina herself, who mistakes Rando for her twin sister that ran away years ago. Now living at Rina's house, Rando is now forced to play the part of Yuna Kurimi while at the same time searching for the real Yuna and attending school. Even with the help of Dr. Manabe, Rando has his work cut out for him!
Boy that has long time crash on girl ends up (through a magical mirror in KnKnH and thanks to a mad scientist in PF) really close to the object of his affections. What happens next is pretty much stereotypical. Boy protects girl from harm's way and at the same time fends off potential boyfriends. I found KnKnH to be more entertaining but nonetheless very similar to PF.
Nothing beats a good school-based comedy with magical elements, and both Kagami and Yamada contain plenty of those with a bit of romance (More prominent in Kagami) and a nice dose of perversion. The magic in both of these series though isn't single-person oriented, as in they need two people for it to take effect. If you have, by some stroke of luck, read either one, check out the other for more of the same style of fun.