Now for my opinion. It all depends on what you are looking for in a main character.
What I like in a character is a true sense of humility and selflessness. A character that is calm and collected in serious moments. One who is intelligent and able to assess a situation and instantly come up with a plan of action. One who hides his own suffering through a jovial mask as to make the others around him feel just a little bitter safer, a little bit more confident that everything is going to be alright. Someone who has worked himself to the bone trying to overcome the obstacles that he needs to face. Someone who I can deeply relate and sympathize with during their trials and tribulations.
Both Ichigo and Naruto are not entirely this type of character.
I love Shonens. I grew up with them, but I do have things about the genre that I don't like. The thing I despise about Shonens the most are their main protagonist. I can't stand the character who is a complete idiot, except when they're fighting of course, and eats a shit-ton of food, and I can't stand a character that needs to be the "best".
Now, what does that mean for these two characters you might ask?
It means that they aren't my ideal character, but it doesn't mean that they have things that I don't like about them.
So if you were to ask me who is the better character: "Ichigo or Naruto?"
I'd have to go with Ichigo.
I think I should explain what I dislike about Naruto first. He's not consistent and he is full of shit.
To clarify, early in the beginning of Naruto, Naruto's past is explained, he's hated and ostracized by his entire village. The thing is, we never see this, more specifically, we never see him overcome his villages' innate prejudge against him through his own actions. We never see him walking down a road getting yelled out by strangers, we never see him be denied food at a restaurant because of who he is, we never get to see his suffering in the present time, only flashbacks. He becomes revered as a hero by his village after his defeat of Pein, but we never saw these same villagers insult or attack previously during the previous arcs. We never saw the effect his accomplishments had on his village, and how that changed their opinions on him.
And like I said, he's full shit. Naruto has a fight with Neji early in the series, and he wins and talk-no-jutsu'd Neji with a speech about how Destiny is bullshit and hard work can overcome those born with natural talent. Naruto is the one that said this. The son of a Hokage, the Godson of Jiraiya the Gallant one of the Legendary Sannin, the reincarnation of one of the sons of the Sage of Six Paths, the host of the most powerful tailed-beast. Neji was right, those who are destined to succeed will. You know who had no natural talent in Naruto? Rock Lee. And guess what he did? Nothing, and it is such a shame because he was a person who got off his ass and worked himself half to death to improve himself. Naruto is not the underdog that he was originally portayed to be, and why I vehemently disagree with the others above in saying that Naruto is a role model, Lee is the real role model.
Now on to why I prefer Ichigo.
Ichigo is a real human being. He's not trying to be the greatest ninja/hero/pirate or anything like that. He has what he wants in life and he makes sure he keeps it. He's not a saint, he just wants to live up to his name, one of the few lasting connections he has to the most important person in his life, his mother. And to make sure what happened to his mother never happens to those he cared about ever again. To make sure those he cares about get to live. And when those he cares about are in danger, he will go to any length to make sure nothing happens to them. He doesn't preach about surpassing the bounds of fate and destiny, none of that shit matters, at least not to him. He reacts to the situations around him, unsure of what the future holds for him, simply put, he's a teenager.
I find this immensely relatable. As a teenager myself, just about to enter my final year of high school, I'm unsure of the future, and that terrifies me. I'm plagued with infinite questions about if my life will work out in the way that I think I want. I just don't know. I doubt myself all the time.
When I see a character who is like me in every way except for the fact that he doesn't hesitate to do what he believes he needs to do, I find that amazing. Someone my age possessing such confidence is awe-inspiring for me.
I can't relate with Naruto in such a way, I don't have a dream to be the president or something, I couldn't care less about "being the best", I'm just a kid and seeing that there is a main character like me is refreshing.
So yeah, Ichigo all the way.