Douglas Booth is our choice for Will.
It was kind of strange for me at first, because he was our original choice for Alec Lightwood. There is one picture where we think he looks like Alec, and one where he looks almost perfect for my brooding love, Will Herondale. These are the two pictures compared:
Alec Lightwood (Left) and Will Herondale (Right)
Does he not look like he's just stepped out of the cover of Clockwork Angel in the Will picture? It is from Great Expectations. A fan put together a trailer for Clockwork Prince using scenes from that movie, as it is set in the right time period, it's so good!