Of course Avengers has much more attractives for the general audience than Potter had , action and super heroes..
but I STRONGLY believe there is more people who read the Potter series than people who read the Marvel´s comic books..
So, in some way, Potter was stronger
And there is another thing.. Marvel has a lot of heroes involved, a lot of autors and producers.. Potter was something made by just one person..
The Marvel universe is much bigger than Potter´s , of course.. but I don´t think it is stronger..