Avengers, hands down. JLA is an incredibly dated book for most of its run. All too often it is workmanlike and almost seems to purposefully avoid characterization.
By contrast, Avengers has some of Roy Thomas' and Steve Englehart's best scripts ever, exciting mixes of characters (Thor, Hawkeye, Vision, Iron Man, Black Panther, Goliath), John Buscema, John Byrne, and some nifty interactions with other teams such as the X-Men, Fantastic Four, Defenders. Oh, and the possibility of an odd duck history was always hanging around the corner, such as the time when Doctor Strange sort-of kidnapped the team to fight Surtur and Ymir.
It was exciting. Unpredictable in a nice way.
What was really odd about the story was Asgardian monsters are running amok on Earth but Thor doesn't show up. At the time he was in space hunting Galactus and fighting Ego, then Pluto, then Him. This would explain why he doesn't help out, except Thor is still hunting Galactus when he does show up in Avengers a few months later. Seems like even if Thor was out of the galaxy at the time that somebody from Asgard would have shown up to fight those two. But Roy may have used that story to say "See, Stan? I really need Thor in this book!"
I think at one point the JLA was purposefully avoiding characterization, under the belief that plot was more important.