I watch and read both, prefer the One Piece manga slightly, and the Naruto anime slightly (would be by a fair bit, but one piece has a better filler situation
One-Piece manga- more character development, a much better war arc, less overpowered (although sometimes the fights have to be scaled down for the weaklings in luffy's crew), the story progresses slow but doesn't drag, interesting and fun even when there aren't fights or developments. one disadvantage is that there are some arcs which are huge let-downs compared to others, like the current hody jones arc. i like the individual chapters in the arc, but when the villian is lame i don't really like the arc. and for every great villian in op there's a sucky one like hody jones, moria or spandam.
Naruto manga- much better villian development, more emotional, better twists imo. i also prefer the fights, they're more fleshed out and tactical and there's a better scale of power levels as opposed to op, although it's getting to the point where strength>smarts most of the time now. what i don't like about shippuden though is that there was virtually no character development apart from naruto, sasuke, itachi, oonoki and other villians, the konoha 11 and kakashi were mostly cast aside.
One-Piece anime- as above, the animation was pretty terrible for almost half the series though, and it took the anime around 200 chapters to finally get a few good soundtracks which messed it up a bit for me. it gets way better after the water7 arc though.
Naruto anime- better music by far, which suit the epic and emotional moments extremely well.good animation imo, i wasn't disappointed in the fights except for the sole naruto vs pain fiasco. but the filler situation is horrible, although most, bar the last atrocious arc, are enjoyable.
