I think Batman will edge out Avengers, but I think its going to be closer than people think. The last one made a ton because of Leadgers death kinda hyped it up. Not to take anything away from his performance(saw it twice in the theater), but that defiantly got the casual fan into the seat the first time.
But I also wouldn't be suprised if Avengers beat Batman out. Marvel has done really well promoting the movie, starting in 08 with Iron man, leading into Hulk, Ironman 2, Thor and then Cap. And this is the first time we really get to see ALL the heros on the screen at the same time. And with Joss Whedon writing and direction its going to be good, the guy hardly ever misses.
I will be seeing all of them. But If I had to pick on...Batman is my favorite, but Ive seen Bats on screen, and its the Avengers first time, gotta support the newbies.