OP here. Thanks a lot for all your responses.
I'm really surprised that some posters think that the original Star Wars trilogy is too scary for 4 and 6 year olds. Having watched them a loop for the last few months, I find them funny, moral and exciting, with just enough danger and suspense for the boys to be on the edge of their seats, but not to be terrified. I think the prequels are more frightening, and I wish the boys wouldn't watch them, but more because they are utterly rubbish films.
So, to clarify my calling my boys a fairly bloodthirsty pair - they love fighting with swords and guns (and this despite my being one of those po-faced mums that swore her little princes would never own so much as a water pistol), they love watching shows like Spiderman, Redakai and Teenage Mutant Ninja Turtles on CITV, which are full of fighting, not to mention Tom & Jerry, and they are pretty unfazed by all the films we've let them watch (which, now I come to think of it also include the first two Harry Potters, but I won't let them watch the others, any more than I'd read them the books, because I think they are too dark).
But they do know that hurting is wrong, that what they watch on screen is not real, and that replicating that kind of behaviour in real life is unacceptable, not to mention well nigh on impossible in many cases given that they don't have access to Acme explosives, superpowers or Pokemons.
Which is why DH wants to take them to see Man of Steel. Why I don't want to take them is because the 12A rating makes me uncomfortable, and also because watching a film like that in a darkened room, with surround sound at full blast, will be far more intense an experience, than doing so in our sitting room on a Saturday afternoon when the DVD arrives from LoveFilm.
My sense from reading the posts from those who have seen Man of Steel is that it probably will be overwhelming and boring, so it's not worth taking the boys to see it, and we'll see Despicable Me 2 next weekend instead. Which is a shame for me, too, because I was looking forward to seeing lots of Henry Cavill. 