Well, I am biased, since I am an usborne consultant myself.
However, even as a consultant, I feel that our books are really affordable, and I was actually really surprised when I first discovered usborne that they did not charge more for their books. If you go to a store like barnes and noble, the books are usually pretty expensive, and many of the books can be quite junky in content and not well made either. The quality of usborne books (physical and content) are excellent, the illustrations are amazing, colorful and draw kids in. Like someone said, we have a great warranty, if you do not like your book, you can return it (within a certain time) to the consultant. The books come with a 1/2 price replacement guarantee. Say, your child accidentally drops the book into a puddle and it gets all waterlogged. You contact your consultant, give her the book, send it back to the company and you can buy a new book (same one) for 1/2 price (plus shipping and tax). The warranty only applies to books that have been purchased through a consultant (a few stores carry usborne books here and there, but not all 1300 titles), since they believe their consultants offer great personal service. Also, what makes these books even nicer is that you can host a party and get free books and half price books! You cannot do that for DK books. Also, usborne does a relatively good job compared to other companies about portraying multiculturalism in their books and they are constantly revising their books to make sure they are up to date. The multiculturalism is a big thing for me. I just had a show today, and the multiculturalism applied to each of my guests. I had parents who had adopted a child from asia (and are in the process of adopting another child from china), homeschoolers who care about diversity in their child's education, bi-racial parents, parents from other cultures, etc.. When I mentioned the multicultural part of the book and one of the HS moms piped in about how that was one of the reasons why she loved usborne books, I really felt that the moms there felt very good about this, since this was something that all of them felt were important to expose their children to.
I think DK books are nice, I have a couple of them myself. However, I have noticed that they tend to use a white background in almost all of their books and after a while their books start to look a little bit generic and boring and I do not think they hold up as well either. Usborne books do a good job of drawing kids in, and I know it sound hokey, but it has been true in my experience. My boys are very active and are the type who do not like to sit still. They will however, sit still when we read usborne books. My toddler will spend over 30 min looking through the, "big book of things to spot," by himself and coming up to me to ask questions about things too. It's just nice to see kids enjoying books. I myself love reading, and I'm delighted that we found a brand of books that my kids enjoy, so much so that I decided to become a consultant.