I remember reading a review many years ago on AICN, who described the difference between Iron Man and TDK, which works right now with the top 2 CBMs.
I am paraphrasing, but here goes.
The Avengers is a great Comic-book movie
The Dark Knight Rises will be a great graphic novel film.
That's the core difference between the 2 film franchises, and there is nothing wrong with that.
All the Marvel film properties since Iron Man (except maybe TAS and Nic Cage's duds) have been adapted like the great comic books in Marvel's history.  The creation of the Super-Soldier, the death of Bucky, the origin of Iron Man, the creation of the Avengers - all from great comic books.  
These movies play like the comics - the hero faces a villain, struggles with an obstacle, a love interest is involved, the hero wins to fight another day.  
It's the same approach that's taken with James Bond, Indiana Jones, The Superman films of the 70's / 80's, and the Batman films from 1989 - 1997.  There's a reason that this approach is so successful - most people enjoy it.  When a film or series is good, the films are brilliant.  The first 3 Indiana Jones films were loved because audiences felt that they could watch that character forever - just like a comic book, the character would not age or die. 
On the other hand, graphic novels take a different approach.  
Batman: Year One, Batman: The Long Halloween, The Killing Joke, Knightfall, The Dark Knight Returns (Ha - notice what these novels have in common with Nolan's films?) - In these novels, story takes precedence over spectacle, and the focus is on the characters in the scenario, and how they develop.  In a graphic novel, characters can change and develop.  Others die and the story changes.  Other characters are damaged, and there's no reset button.  In a graphic novel, a character can age, change personality, die.
An example of a film series following a graphic novel template is the Godfather trilogy.  Aside from a weak ending, the 3 films focussed on a central character and his development from a young innocent to an old, lonely villain.
Both comic film types are absolutely fine.  I love Indiana Jones and many James Bond films, and Captain America was probably the best comic book films I saw last year, but I prefer the BB / TDK / TDKR approach.
You cannot compare the 2 types of film franchises, just because they are based on comic book properties - Its like comparing The Hunger Games and the Godfather because they were both adaptions of fictional novels.
It doesn't matter about the rating (it'll be about the same for both), the audiences will be different.  To put it into context, kids, parents and families will make up the bulk of The Avengers audience in terms of gross.  For TDKR, it will be young adults and teens.