Advances for most authors are based on how many books the publisher thinks will sell. The big blockbuster advances are for celebrity authors or the rare novel with lots of interest.
In my opinion, it pays the author more to have a higher royalty rate. If the book sells as many copies as projected, you'll get your money either way (the advance is an advance on royalties, so any lack in the advance will be made up later). If the book sells better than expected, you'll make more on the backend with a higher royalty rate.