Cree uses "bin numbers" to identify which generation of technology a particular LED uses - each bin number is a letter (currently letters P through R) followed by a number (usually 2 through 5). So a P4 LED would be older technology/less efficient/less bright than a Q2, and a Q5 would be newer/better still.
The actual difference in output between these bins is small, especially to the eye. You wouldn't be able to visually see a difference between a Q2 and Q5 in the same flashlight, though scientific equipment would bear out that the Q5 is roughly a dozen lumens brighter. But if you've got to have the latest and greatest, the Q5 is the obvious choice. (Just keep in mind that technology moves quickly, we're already seeing the all-new R2 bin Crees in a few flashlights, and they'll be more prevalent over the coming weeks.)
