If I had absolutely no concept of what anime was, and was introduced to these two series today, then I'd say Attack on Titan is better. The plot is more complicated, with a far better story structure, greater character development, and an overall greater menace to the 'villains' since there's no magical plot device to reset everything like in Dragon Ball.
With that said, I am old and from my perspective as an anime fan, I'd rate DBZ as better simply because it's had a far more drastic impact on so many people. I could walk up to any moron on the street and ask them if they'd heard of Dragon Ball, and they'd probably say yes. The series, despite being basic as all hell, basically created anime culture in the US.