One Piece is about Monkey D. Luffy in a quest to find the One Piece and become Pirate King to fulfill a childhood promise, Fairy Tail is mostly about Natsu Dragneel in a quest to find his missing father. The first problem I have with Fairy Tail is that Natsu is the only one who still have a goal, his friends have either complete there goal doing there arc or in the second chapter. Lucy Heartfilia's goal was to join Fairy Tail and she did it in a chapter. Gray Fullbuster's goal was to kill Deliora who destroy his village, but it was change to make friends. Erza Scarlet's goal was pretty much save Jellal and her friends and she did it.
To me, there goals just come to them way to easy. I feel that main characters goals should be completed near the end of the story, not just in an one arc. It would be ok if they get another goal.
In One Piece, Luffy and his friends have goals that will last them for the entire series and also some of there goals have change. Nami's goal is to create a map of the world, which no one in One Piece have done before. Zoro's goals is to be the best swordsman in the world and help Luffy to be Pirate King. Usopp's goals is to become a brave warrior and visit the land of the giants (funny thing is that he basically completed his first goal without even knowing it) Sanji's goal is to go to the All Blue. All there goals are impossible to reach or hard to achieve, making there journey interesting.
Fairy Tail's arcs are not memorable, there no part of the arc that have a "this is Fairy Tail" type of moment. (It just doesn't stand out to me) Like in the Fighting Festival Arc, in the end with everyone sending off Laxus. It didn't impact me, since Laxus was trying to kill everyone in the town and he was acting like a jerk all the time. The arc didn't built up to or give me a reason to care for the scene
With One Piece's arcs, they built up to the scene (sometime before the arc). Like in the CP9 arc, with the Going Merry send off. It impact me, since I can understand how they feel. The arc show me how important the Merry was to the Strawhats.
Well, I have other reasons why I prefer One Piece over Fairy Tail, but these are the major ones. Both are good series, but I feel Fairy Tail is lacking. I have completely stop watching and reading Fairy Tail, I plan on one day coming back to it (but its extremely unlikely).
Well, I got that off my back (sorry for any typo), so which series do you guys prefer