They are both reverse harem-situations.
In Shiawase Kissa Sanchoume (Happy Cafe) there are three guys who like the same girl, even though one is oblivious where as in Ouji To Majou Himegimi there are six guys who like the same girl.
In both manga's the guys are all good looking and they are both very funny manga's.
In both the main character stays oblivious to who she truly loves for a lot of the manga. In Shiawase Kissa Sanchoume the main girl is short and cute where as in Ouji To Majou Himegiri the main girl is the Princely type.
In my personal opionin, if you iike one, you'll like the other.
Kyoko moves to the big city with the prince of her dreams Shotaro; he wants to make it big in the entertainment business, so she works hard at many different jobs to support him as he achieves his dream. However, one day, Kyoko accidentally discovers the horrible truth: the love of her life thinks of her as a 'plain and boring woman!' Outraged, Kyoko swears revenge - she will make it even bigger than Shotaro in the entertainment world. The only question is, how will she fulfill her desire?
I can't say that these stories really have much in common, beyond both having really interesting characters and stories. If you're reading one and want something equally fun to read, I'd recommend checking the other out. Uru and Kyoko are both really fun, quirky main characters, and Shindou, Ichirou, Ren and Shou are all attractive male leads as well. Neither of these mangas ever get overly sappy or trite either, which is something that not many shoujo mangas can claim.
Hikari Hanazono has never been able to beat Kei Takishima at anything. At six years old the two were introduced by their fathers, and Hikari promptly challeneged Kei to a wrestling match, believing she'd easily beat him. However, she lost, and since then she's vowed to beat Kei, following him wherever he goes. Now fifteen years old, Hikari attends the super-elite school Hakusenkan where she and Kei are the two top students; Kei holds first place, while Hikari is second. While Hikari considers Kei her special friend and most importantly her rival, Kei is secretly in love with her. Will Hikari ever beat Kei, and Kei's love ever be requited?
Both Hikari and Uru are incredibly slow at realizing their feelings. Though unlike in Special A Shindou doesn't realize his feelings straight away. In both Special A and Shiawase Kissa Sanchoume there is a girl character that is protective over the main character and dismisses all the guys. They are both really funny and I loved reading both of them.
Ageha holds two things dear to her heart: the series of purses she designs and gives away, and her overly-idolized elder sister. When her sister announces she is pregnant and is going to be wed to her loser boyfriend, Ageha couldn't be more shocked and refuses to accept the news. Meanwhile, Ageha and her friend bump into two men at the mall who seem very interested in her purse series, but little does Ageha know she will meet them again quite soon at the very establishment her sister is having her wedding dress custom made: Velvet Blue Rose! Now Ageha must prove her sewing competence in the ultimate test: making her sister's wedding dress!
I loved both these magas! They are similar in the way that the main character get a part time job with two guys, and she ends up liking the guy in charge. :) There is some rivalry as well. Happy cafe is really cute and the characters are really funny.