Both shows are fun and entertaining in their japanese versions.
Both shows are sheer and utter sin in their american versions.
I like Luffy and his dreams of being a pirate king.
I like Naruto and his dreams of being basically ninja king.
But Naruto has a more complex world and back story. As the story progresses, there are more elements that would engage the reader.
One Piece has less predictable plots, and from what I've seen there isn't as much stalling and filler. The battle against Orochimaru in Konohoa took forever. I almost lost faith in the series because of that. And I stopped watching around episode 150, but my friends tell me that it's not worth trying to get new episodes.
So while I think that Naruto is initially a better series. One Piece shows more promise and will not get boring after 150 episodes. In fact, you don't even have the whole team until many episodes in. While in Naruto, most characters are present from the beginning, and new characters are added as needed. It's a little too obvious.
Though Naruto is a better character than Luffy, IMO. He has more depth to him, and shows more emotions. Luffy is impossible and almost nightmarish in that you never truly know what's on his mind. He's kind of creepy. He has a one tracked mind and it reflects on his character making him only 2D.
Naruto on the other hand has fears, hopes, desires and other things. All aside from one day becoming hokage. Naruto has reachable goals, and one big future goal. He wants to be happy, have friends, and basically be loved because he never knew love...and was hated his whole life. He has reasons behind his goals.
Luffy just looked up to this pirate and wanted to be like him.
For that reason, Naruto would more likely reach his goals, than luffy. Luffy, being defeated again and again would soon just get pissed and give up or die. Naruto has to keep fighting because he is his dream, and people are counting on him.
Naruto would also beat Luffy in a fight. Because if Naruto was in Luffy's world, he could just knock luffy into the water. Naruto can walk on water, so he'd win. If Luffy was in Naruto's world....Naruto would have the advantage of trees and stuff for ninja tricks. And Luffy's stretching would be minimized by the lack of space.
So draw your own conclusion but I think Naruto is the better show.