I'd say it depends on what you're trying to achieve, exactly.
Do you have some specific reason for why you want a new trailer now? Is there some specific marketing campaign you're planning to go with the new build's release? If so, then sure, make the trailer now and push back the release if you have to.
If not, though, especially with the new build coming so soon, I'd think it would be better if you don't rush the trailer. Get the new build out, and have the trailer contest end a couple of weeks after is release. A week or so for people to play the new build, and a week for them to make trailers.
Look for a trailer that's general and captures well the essence of the game, but doesn't focus too much on specifics. Then you can use that to get people interested in general. Following that, you can start making "teaser" trailers that focus more on specific features when you make updates with major content (*coughHeroescough*).
Right now, there's an influx of new players due to the alpha bundle. Aim to release the new build right after that ends, so you have all these people who've just discovered the game and are still excited about everything, and they get to play the game as it is now, make some opinions of it, maybe start thinking "Damn, I should go on the forums and suggest they introduce priorities! I bet nobody's thought of that, I'm brilliant!". But just before they do, the new build comes out, and they're like, "Oooooo, new stuff! And priorities! I'm psychich!".
So there's a quick overlap between seeing the old (present) build and new (next update) one, and players get to really experience the transition. If you release the new build right away, people who buy the bundle near the end won't be able to experience that. And given this recent influx of new players, I don't think you need the publicity the trailer will give you right away.
So now the new build is out, and you "officially start" the Trailer contest (people could have started before, but you'll want some of the new stuff included). Let that run for 2-3 weeks. I'm assuming that Heroes are coming next update after... so now you get the main trailer a few weeks before heroes show up. You get new customers because of the trailer, and just as those new customers might be starting to lose interest a bit, you come out with a "Finally! Heroes!!" teaser-trailer, and everybody goes wild.
At which point I go into hiding because the bug forum gets spammed with a million "OMFGWTF MY CITIZENS ARENT EATING OR MOVING!!1!" threads, but you don't care, because by this point, you're rich... >.>
TL;DR Wait a bit for this whole trailer contest thing. Bring out the new build, and let the trailer be influenced by that. Look for a trailer that's general and captures the essence of the game, then make a new mini-teaser/trailer for Heroes.