As a general rule, you should definitely not install more than one codec for any particular file format. Both the FastPictureViewer Codec Pack and the Microsoft Camera Codec Pack are fully uninstallable and
must be uninstalled (and the computer restarted) before installing the other, or any other similar codec for that matter.
To make any comparison valid the system's thumbnail cache should be cleared before switching codec packs, using the built-in Disk Cleanup utility (after uninstalling but before rebooting). From my initial observations, the FastPictureViewer Codec Pack is still the speed king by a wide margin (most strikingly in Windows Photo Viewer, perhaps 3 to 5 times faster) and the FPV Codec Pack supports far more brands and models than Microsoft's, including all the latest cameras.
Both are listed as "
Tested and Recommended" by the Microsoft Research team behind the Image Composite Editor.