The EagleTac P20A2 and P20C2 are essentially the same light, but they use different batteries and have different outputs.
The P20A2 runs off of two AA batteries and has a max output of 272 lumens. The P20C2 utilizes two CR123 lithium batteries and has a max output of 524 lumens. Functionally the two torches are the same, but the differences in the power source and LED means that the P20C2 will be brighter and also run longer than the AA version. Both lights are insanely bright for the price, which makes them a smart choice someone wanting a high performance light at a reasonable cost.
Watching the video below will give you a better idea of what to expect out of these lights. Enjoy!