First of all, both of them are very good books. They are the only books I've seen that focus on software development methodology instead of PHP language features. Even an experienced PHP developer should take some time to read through them (or at least one of them).
Both of these books are organised in three parts: 1. PHP Class, Objects, OOP design; 2. Design Patterns 3. Testing and other practice such as version control, refactoring.
For people who have any experience of C++ or Java, the first part of the books may be safely skipped. However, pure PHP programmers, especially those still using PHP 4, better read through the first part.
Second part of these two books are most valuable to PHP developers and worth careful reading. Each book introduces some design patterns. Some of these patterns are introduced in both books. The others could only be found in either one of them. PHP In Action uses one chapter to explain six design patterns, the other book spends 5 chapters in presenting 10 design patterns. Personally, I think the second books presents design patterns in more details.
Both books have chapters about database patterns, unit test. PHP In Action also introduces some concepts of refactoring while the other book uses a whole chapter to show us enterprise patterns and it also introduces other PHP development practice such as CVS, Pear, etc.
As I said, both of these two books are good and worth your reading. However, if I have to pick up one winner from them, i will choose PHP Objects Patterns and Practice. Compared with its competitor, it digs deeper and explain more details.