I think Superman probably is more relatable in terms of life experience; all of us have felt like outsiders, all of us have tried to be normal and have to balance that with standing out to behave in extraordinary ways. Superman also is from the middle class at best, his life growing up is very "American." (He even says that in the movie ("General, I grew up in Kansas. How much more American can I be?")
Few of us have had our parents murdered, and few of us are upper-class billionaires. Batman is "cooler' on some ways, but not necessarily more relatable.
Superman's always had a lighter touch in American cinema and isn't touched by darkness, at least in the TV/movie media. And this was also Reeve's iconic role, and he is somewhat of a legend due to his struggles from being paralyzed and from his expressed character. I have trouble believing those things aren't playing into some of the critical response. Note how much higher the userbase response is.