A123 cells are a particular type of LiFePO4.

The A123 cells use nanotechnology to significantly improve the ability of the cells to deliver high currents (up to 50C) whereas a standard LiFePO4 cell is rated to about 3-5C.

I use A123 cells in my gas planes where the large number of hi-torque digital servos can produce peak current draws of up to 15A or more but on my smaller models regular LiFePO4 cells work just fine. I fly a lot of 3D stuff with .46-sized profiles and even with the very large control surfaces plus large throws I've never had a problem running 5 standard servos and a 2.4GHz receiver on a regular LiFePO4 pack.