Here's the thing.
DmC is out this year, when they announced that they did well into 2012 and if they were going to delay it they would've known by then. That measn that they have enough material to show, even if it'd all be patched together just for a trailer.
E3 is in June, half way through the year, which means that even if the release is in december they should be able to give us an exact date by then and six months before release would be a good time to let us know everything we should know. On top of that E3 should be the event where they really let it all out, it's nice and global, pleanty of attention to be given.