MeruPuri is a short series consisting of only 4 volumes and written by Hino (also author ofVampire Knight). Tail of Moon is quite the opposite in length, comprising 15 volumes.
There were three main factors I considered in this Shojo Showdown! - plot, characters, and X factor.

Bottom line: It's a tie. Both manga have interesting storylines and a theme of love.
-MeruPuri-- The main characters are Airi, the story's heroine, and Aram, a prince. Over the course of the series, they develop enough for the reader to develop an understanding and empathy for these two characters. Because the series is so short, though, the other characters are not as well-developed. There is an older brother to the prince (comic relief), a servant to the prince, another prospective love interest for Airi, a fairy, the queen and others who play minor roles, but nothing like the main two. The author does a good job in developing the relationship between Airi and Aram, and raising the level of romance as the prince gets older.

Bottom Line: -Tail of the Moon- wins. The characters are developed a great deal more than in MeruPuri-.
X Factor (that extra something):
-MeruPuri-- For me, the X factor in -MeruPuri- was the mental creep out. Airi is 15 and falls in love with a 7 year-old. He transforms into the body of a 17 year-old which is fine, but still has the mental capacity of a child, which, as I said- creeps me out!
-Tail of the Moon-- Ninja training!! It was awesome reading about ninja life, training and assignments. It makes you feel like you are there. Also, the ending! I’m conflicted about the ending of this manga. Maybe it’s because I didn’t want to see it end after 15 volumes, but I did not like the way it was concluded, though at least it’s memorable. I can’t remember how half of the manga I’ve read terminate.

Bottom Line: MeruPuri -…definitely loses in X Factor. -Tail of the Moon- wins.
Winner: The winner of Shojo Showdown! is -Tail of the Moon-!