I may see where you are coming from as both authors and these books have great reviews. After reading a few pages from both books, a number of reviews on amazon and a few google searches later I've come to ask myself if I wanted to know more about packet analysis or the wireshark program? Here is my approach. If I wanted to learn more about the wireshark program I would choose Laura Chappell simply because she founded Wireshark University and she does a step-by-step walkthrough of the labs. If I felt comfortable using Wireshark or Microsoft Network Performance Monitoring (free tool from Microsoft) I would choose Chris Sanders as he expects you to know how to use Wireshark and dives into more details of packet analysis which is ultimately what you want to do if you are looking at both of these books or using a packet capturing tool.
Level 1 book w/minimal to no Wireshark experience and just starting out with analyzing traffic.
--Laura Chappell / Wireshark 101
Level 2 book w/some experience using Wireshark and capturing traffic that expands on analyzing traffic.
--Chris Sanders / Practical Packet Analysis
Possibly other helpful links:
Wireshark University (Founded by Laura Chappell)
Comptia Network+ (Specifically chapter 6, learn how the protocols)
Sorry for the lengthy post but some may find it helpful while others will tell you there's a ton I left out. =)
