This debate has been going on between me and my friends (and others, i'm sure) forever. Let's finally settle it.
I vote DBZ because I got way more emotionally invested in the characters.
However, Naruto has the benefit of being more modern, and has edge in plot development. The were some episodes of DBZ that were COMPLETE waste of 21 minutes, especially after waiting forever for the fights to begin.
I gotta say Naruto Shippuden. The story line is way more complicated and entertaining. Although the plot seems a little dragged out, I find Naruto Shippuden the superior series.
As for DragonBall Z, the series should have ended at the Cell Saga. Goku's Sacrifice was the perfect ending and the series should have ended on the focus of Gohan being the main character. Buu Saga and Fusion Saga was just ridiculous.
Story: Naruto Shippuden. NS has the kind of story I like more which involves a consistent big bad that needs to be defeated before the story is fully resolved. In Shippuden you start out with the Akatsuki as villains and throughout the entire series the Big Bad was tied to the Akatsuki whether it be Pain-Nagato-Tobi-Madara. It all evolved out of one organization so you could feel the logical progression of events. Compared to DBZ where it's "Here's a bad guy, he's dead so go after this next one you knew nothing about". This is what I personally care about in a story, I like being able to make sense of the progression. Keep in mind I'm also rejecting next to every filler in Shippuden outside of the character backgrounds.
Characters: Split. I like DBZ's characters for nostalgia and their sheer power. It will remain one of my favorite fighting anime but when you think about it the characters don't have a story based around character depth or backstory besides maybe Vegeta who has to battle his History and Pride. If the show Stopped at the Frieza saga I would have given DBZ this category and Story as well. For NS the characters are much more important to the story. The Naruto/Sasuke storyline bores me but the Kakashi background (filler) and Sasuke/Itachi storyline was good. There's a lot more likable characters in the background and foreground I think which makes it easier to connect with one. Especially because of their varying personalities. Whereas in DBZ your personalities are either peaceful fighter or prideful fighter.
Animation/Art: Naruto being a more modern series has better animation and art. If not for nostalgia one could get thrown off based on the look of DBZ. It's an 80s-90s series (I think). Plus the powers are more varied and not just a bunch of energy beams vs energy beams that keep making the same sound effect. It's a higher quality production so it's easier to watch.
Guess I have to go with Naruto > DBZ. It surprised me because going into the topic and before thinking about it I would have gone with DBZ
Both are great in my book, but at the end of the day I say Naruto shippuden since it stays good with story and characters. DBZ, well the characters mellowed out to something bad and I didn't care for it after the cell saga.