Iron Man 3 > Man of Steel
Watching Man of Steel, it is hard to believe that the guy who directed it also directed one of the best comic book films of all time, Watchmen. Peering further into the matter, it becomes obvious that the problems of Man of Steel are not Snyder's fault, but the studio's. They never should have green-lit the piece of garbage that Goyer supplied. The best way to describe it is to use a Hype connection: it reads like an answer to one of those threads named "How Would You Have Done (Film X)?" In this context, it was, "How Would You Have Done Superman II?" The result is an infuriating mixture of unoriginal (Superman II) and fan-fiction extreme deviation (Superman killing Zod.) I can overlook the inconsistent cinematography, but, not the script.
Snyder has earned his place in Adaptation Cinema: he did a fine job with Guardians of Ga'Hoole, a glorious job with Watchmen, and managed to make a Frank Miller story a part of popular culture. Goyer, on the other hand, seems to be going the route of Jeph Loeb: he hit his peak with Batman, but is on a downhill slide, with no end in sight. Loeb can be excused because his son died---but, what's Goyer's excuse? It's the writer's equivalent of Ridley Scott and George Lucas: they made the studio so much money, that nobody bothered standing up to them.
In context of Justice League, if Snyder is directing, I hope it is with a different screenwriter and director of photography/design team. Justice League needs to have visual richness and intricacy of Watchmen; Snyder's already done one victory with his casting (everyone in MoS was perfectly cast. Not one bad note.) I just hope he's given some adequate material to work with, so that DC level the playing field with Marvel.
Now, I recognize that the two films are not being filmed back to back, and am commenting on hypothetical scenarios, like everyone else.