In regards to the PCIII usb or RT. The difference in using PC USB over using Race Tuner is that the Race Tuner is capable of doing Volumetric Efficiency which the PC can not do as well as independently working with each cylinder.
That means you can set your air / fuel to what you want then adjust the efficiencies of the cyclinders independently. That means if front cyclinder is running leaner than rear you can fix it where you cant on the PC. The PC can only do timing and A/F.
If you come off your base map you can not increase the efficiencies of your cylinders so your going to find that there is a max you can achieve hp/tq. I like the power commander in some of the things it can do, but with the newest software released by HD software versions 4.50 and 4.12 you can add a fuel pump and a decell fuel reduction so now the PC has really no benefits except that you can find more tuners to work on it.