Similarities:
- Both are "Panty Fighters" with high amounts of fanservice
- The lead female character is a very skilled fighter
- Both involve a competition
- Both have at least 1 apparent death scene
- Both have a grand prize for winning the competition
- The main protagonists of both are entered against their will
- The losers of fights are taken away by the organizations in charge of the competitions
Differences:
- Sekirei is also a harem anime; Platonic Heart is not
- The main character of Sekirei is male; the main character of Platonic Heart is female
- The ending of Sekirei (the first season, anyway - I have yet to see the
second one) makes sense; the ending of Platonic Heart does not
- Platonic Heart fails to deconstruct the Panty Fighter genre (while simultaneously being guilty of many of the things it's trying to make fun of); Sekirei doesn't attempt to deconstruct anything
- The main female character of Sekirei is skilled because she has super powers; the main character of Platonic Heart is skilled because her mother taught her how to fight
- Sekirei has a good ending (in the sense of how it's executed, and the quality), while Platonic Heart has a bad one (the only good thing about Platonic Heart's ending is that it's the last part of the anime)
- The fights in Sekirei have more intense fighting, whereas the fights in Platonic Hearts have more fanservice (although the fights in Sekirei do have a good amount of fanservice)
- The fights in Platonic Heart have a bloodier ending (with one exception); the fights in Sekirei aren't as bloody
- One of the characters in Sekirei comes back to life after dying; all the deaths in Platonic Heart are fake
- Most of the things that Platonic Heart does wrong are done right in Sekirei read more 
