I thought The Power of Six was slightly better than I am Number Four. I am still not too fond of the writing style, especially now that it's written in alternating perspectives (which threw me off at first). 
 So you would think that The Power of Six would be about Number Six, or written in her perspective but nope! The Power of Six alternates between the perspectives of Number Seven (aka Marina) and Four. Not that I mind, though. I really like Seven/Marina. Also, we get to see how both Seven and Four perceives Six and it just makes her that much more of a badass. 
 The Power of Six also made me realize just how much I dislike John/Four. He's so annoying, omg. How is he even still alive???! He's selfish and impulsive. Not to mention that his powers are really lame compared to everyone else.