I think Preston's book is going to be a bit larger.
I haven't read Creating the Enterprise yet, but it looks like there's a fair bit of autobiography on either side of the chapters about TMP. Lots of positive quotes from some Trek tech fans, so I'm reasonably optimistic.
The Cushman book is a revision of the first book, but there are still a fair few typos (not nearly as many as some small press books I've seen, though). As for errors, I don't really know, but there is a lot in here I don't remember reading before, especially the info on previous versions of scripts. But it's a long time since I read any TOS nonfiction, so he may be repackaging stuff -- but given the size of the books, I think he must have a considerable amount of new info here. Cushman supposedly had a lot of studio documentation that hasn't been widely seen, and he also draws on interviews he's done and on a wide variety of published material. If you're not sure about the Cushman books and you don't mind ebooks, you can get the revised first for a lot less than a print edition. I will say this: the book has had me really wanting to go back to my TOS DVDs, which had been gathering dust lately. I've rewatched a few eps. That's a good thing, right?