With 3rd person, you're the movie director. You're a master of sleight-of-hand, showing the reader this scene, withholding that bit of information, moving from person to person, building tension, jumping across continents, moving through space and time. You are the Author. You can do anything you like.
1st person, on the other hand -- and I know 9/10ths of YA is written in 1st person -- is messy. You're in the middle of everything, plopped down in your character's mind, wading through the long grass of distraction. You don't know the half of what's going on. Forget the well-tempered sentence: you drown in a sea of "anyway," "sort of," "kind of," and "I mean." You're not sure when to stop and so you keep going on and on, until you're sick of your character and just want out of his/her head.
OK, maybe you can tell I'm just a little bit prejudiced in favor of 3rd person. Frankly, I think that 1st person is (sometimes) a writer's crutch. It lets you do more with less. But before anyone who loves 1st person gets upset, let me just say that I'm willing to look at both sides of the subject:
3rd person is cold. It's remote. You're up in the sky, looking down on the characters. Good luck getting close to them as they wander through their lives while Mr./Ms. Author , who seems to be on a serious ego trip, tells you all about the history of some other place or person.
1st person is focused. It's you, telling your story. It's you living your story. You're witty, intelligent, sarcastic and flirty all at once. Everyone loves you. You have VOICE.
My challenge this time around, since I'm writing first person from two different characters: make them sound different.