In short, I think Naruto is a more realistic character and therefore easier to relate to. Sure he might have been stupid in Part 1, but he still got upset and depressed, which is something that I feel like I could relate to due it being a typical human trait. Luffy, however, is always overly happy and his stupidity is always exaggerated. Hell with Luffy one of the only times I have been able to relate with him is when Ace died, which really shows why I care about Naruto as a character more.
Also, after the timeskip Luffy got very strong, but in terms of personality he didn't develop in the slightest. Naruto didn't become overly strong during the timeskip, but he became a hell of a lot more mature than he was in Part 1. When Naruto came back you could tell he wasn't a child anymore, Luffy is 19 and he still acts like a child. Hell the only reason I can think of that would validate Luffy as a character is if he was retarded (no offense to anyone).
Tl;dr
Naruto = More realistic = Easier to relate to
Luffy = Unrealistic = Harder to relate to
EDIT: Also, I think Naruto's character is a lot more developed than Luffy's. We know nothing about Luffy, meanwhile we know everything about Naruto. One Piece was 300-400 episodes in and we only then found out that Luffy's grandfather was a Marine war-hero, and that is father was a revolutionary. Hell it wouldn't be that unbelievable if Luffy some time says "Oh yeah, my grandfather on my mother's side is actually Kaido, and my mother is actually a former Yonkou" or something along the lines of that.
