Gabbe, by comparison, is easier to read and covers essentially the same material. This text is written in a way that that I feel you don't have to be an OB Geek to follow the various points. This is NOT to say that Gabbe is "OB for Dummies" or a cliff-notes version of an OB text: As Department Head of OB/GYN at a Major Hospital, I find Gabbe both authoritative and complete.
Cutting to the chase: Gabbe is easier to read than Williams, and if you need info more in-depth, you should already be a fellow of the American College of Obstetrics and Gynecology anyway (or at least a junior fellow), which means that you're accessing the ACOG website on-line anyway.