I think I read about three or four JLA issues, so definitely Avengers.
Justice League of America.
I used to go back to the Justice League whenever the Justice Society from Earth-2 crossed over. I don't think that The Avengers did that kind of thing very much. A sense of history is nice,
Didn't much like it when someone at DC ruled that Justice Society had to be drawn in "cartoony" fashion, but early on that wasn't a problem..