I pick Naruto, spoilers ahead so don't read unless you have watched a fair amount of both animes.
One Piece is too straight forward so far that I have seen. I'm still watching One Piece and I'm on episode 191 right now, the fights are cool but a bit too straight forward. This guy ate this fruit and that guy ate that one and those are their set powers for whole anime so far, I know that their powers will be stronger in future episodes but the basis is the same, Luffy is a rubber man and he will continue to be one for the next 400 episodes with a few added perks.
In Naruto the characters continually grow, become stronger and learn new jutsu, not only the main characters but all of them, you actually watch them grow. It appears to have more complexity and I like how the whole chakra system, elements and bloodlines are general knowledge to anyone that watches Naruto. Never know what the characters will do and how they will end up winning the battle, well except when Naruto is rasengan spamming its kind of obvious how his fight will turn out sometimes. Although he has shown improvement since he learnt sage mode and fought with pein that was a really unexpected battle.
Also people actually die in Naruto when they screw up, in One Piece no one seems to die, they always survive and come back from the brink of death. In Naruto when the 4th hokage got his ass kicked he actually died, there was no doubt, the village had a funeral and everything and he gave his life to protect the village from Orochimaru. There is actual drama involved.
In One Piece the bird dude in Arabasta carried a bomb up into the sky to save all the soldiers and he blew up with it in his hands but about 4 episodes after you see him good as new with a few bandages on him and crutches and I just thought "what the hell this is such a childish happy ending".
Basically Naruto is more of a dramatic anime/manga that holds some realism to it while being a complete fantasy. People die, all the characters have their own story and then you have this amazing clash between the Naruto and Sasuke fanboys/girls lol. Many more reasons but I'm already rattling on long enough.
One Piece has some good humour, Luffy is more of a dumbass then Naruto which is hard to believe, but as I said he doesn't really develop like the characters in Naruto did. Luffy starts off episode 1 with being a rubber man and here I am at episode 200 and he is still the same and fighting everyone exactly the same way by just charging in with rubber moves. Naruto on the other hand was useless at the start, then he learnt his first techniques such as shadow clones and after moved onto training his chakra, getting a new master, more control over foxes chakra, rasengan, spams his rasengan for a long time and then gets into sage mode and improves on everything makes up new jutsu's and it keeps going still in the manga and this is just ONE character, I haven't even mentioned Sasukes improvements or Sakura's, even the side characters like Neji, Kakashi, Gaara and Shikamaru.
Also not worth watching just for the occasional laugh when I can watch Beelzebub and laugh my ass off 24/7.
My opinion and what I think, justification is there so flaming is futile. Maybe I may have been a bit more lenient towards Naruto but that just shows I like it more. My opinion has a chance of changing if I get to the latest episodes that are apparently so amazing but for the characters to remain the same until episode 200... Very slow development and its not like there are a lot of characters to show either.