Yeah that hits the nail. The Justice League is a simple toolset where every tool has an obvious purpose. The Avengers are more mixed up.
There's also the fact that the Justice League in addition to being a superteam is actually a superteam composed of its biggest and most popular and beloved characters who often fight villains combined from their rogues gallery and others created specifically to challenge them all. Batman, Superman, Wonder Woman, The Flash, Green Lantern are DC's biggest heroes (add in a token Martin Manhunter or a Hawkperson or two or three). So it actually represents the best and brightest of DC.
In the case of Marvel, the fact is the Avengers while having a somewhat lofty (and largely verbal and official) position as this big super-team for the most part rarely featured its biggest, most popular and beloved characters. Spider-Man, the X-Men, Fantastic Four were historically Marvel's biggest characters and they weren't regular or official Avengers a lot of the times. The three villains with the most appearances in Marvel Comics -- Dr. Doom, Magneto, Green Goblin -- aren't primarily Avengers villains for most of its history. Now some fans are gonna go -- But Bendis -- and I'll point out Bendis was explicitly trying to make The Avengers more like the Justice League and actually incorporate their most famous and beloved characters in the line-up.
There are team-ups and parodies of Fantastic Four across the board, of the X-Men too, and of Spider-Man. So those are the distinct cream of Marvel's crop, at least over a longer period of time.
So the Justice League lend itself better to being the actual pantheon of a superhero universe as a model, more than Avengers do.