The FX3 series is more powerful, manages more of the USB communications in hardware and via an RTOS, and supports SuperSpeed. John Hyde's SuperSpeed Device Design By Example book would be useful. You can see the first pages on amazon.
Though less powerful, the PIC32 series is fine for many uses and the provided firmware lets you see how the USB communications are being managed.
I haven't used the development kits so have no comments on those specifically.
Both companies have user forums.
USB Complete Fifth Edition is much more current on both hardware and host software.
http://www.janaxelson.com/usbc.htm