You have picked two of my favourite machines to compare!
I own both, and enjoy each of them.
They are very, very different games.
ACDC Premium has flow, not so Avengers LE.
Trapping the ball with the flippers is essential with Avengers, less so on ACDC.
ACDC is the easier game, Avengers takes greater skill and is much harder.
ACDC has 4 ball multi-ball, Avengers 6, but 6 adds little to the game.
I keep going back to both, for different reasons.
I enjoy ACDC more the longer the game goes, but if Avengers forthcoming code's upgrade will include more excitement at the Assault on the Helicopter mode and beyond, then it might achieve greater heights than at present.
ACDC is red, my Avengers is blue (the Hulk is my least favourite Avenger so I went for the better looking machine despite what others think, although I am pleased Gary Stern prefers the blue one), both are great to look at, both are keepers.

If I was only able to keep one of them, which one would I choose? ACDC, no question, but that's because it is a truly sensational game.

The bigger question is deciding between ADCDC Premium, Avengers LE,

and …

Star Trek LE!

Agreed! Buy both and help push back "Peak Pinball"!