I've just sent in the text for my appearance in John Scalzi's special feature The Big Idea. I could have done it on either GCA or Threshold, and John did advise me that in general he sees better results for the book coming out closest to the Big Idea feature (which would be Threshold).
I decided nonetheless to focus on Grand Central Arena, for two reasons:
First, speaking purely from a marketing point of view, Threshold already has considerable boost behind it; it's a sequel to a book that did quite well (more than earned out in HC and sold a lot of paper copies) and was quite well received, and it's got a vastly bigger SF author's name on it. It's GCA that NEEDS publicity to pull it up, if publicity can do it.
Second and, I think, more importantly, there's so much more Big Idea in GCA than in Threshold -- and the biggest ideas in Threshold are really from Boundary, for the most part; moreover, the big ideas in Threshold and Boundary are only part mine -- the other part being Eric Flint's.
So I've written a piece on Grand Central Arena's Big Ideas. I could have gone on a lot longer, but I also didn't want to go to a boring length. If I get enough response from it, I may expand on that essay; it was fun to write (but difficult; I spent two hours on it and managed about 1100 words -- which is about half my usual writing rate! I was writing, then deleting, rewriting, deleting again, something rare for me.).