Mike,
The P3 and the P2 function the same as far as braking performance is concerned. The P3 does present a bigger display but that's about it. The P2 and the P3 both have self leveling and can be mounted at pretty much any level. They both have electric over hydraulic brake capability. The big difference comes in the diagnostic displays.
If there is a problem somewhere in the trailer braking system both controllers will tell you what's going on with diagnostic information. The P2 will give you a code that you will need to know or keep your list of codes in the glove box. They come in your instructions.
The P3 just flat tells you what's wrong. No code needed.
My brake controller is the first generation Prodigy, no electric over hydraulic and I have a more limited range of angles I can mount my control....that's about it. Everything else is the same. Oh, and I have a red display instead of blue.
Some people do not like the big display of p3 some do. I would not trade a P2 for a P3 as the display is the only difference.
Thanks!
Jeremiah