Prodigy P2 and P3 are functionally the same. So if you are looking for the best bang-for-the-buck, get the P2.
P3 has a more user-friendly interface, but you pay more for it, without any functional benefit.
I have the original Prodigy (on it's 14th camping season!), purchased before the P3 was available, but it's nearly identical to what they now call "Prodigy P2". Given the longevity of my Prodigy, if I were purchasing a new controller, I'd spend the extra $20-30 for the P3. But you really can't go wrong with either one.
thebrakeman ('70), DW ('71), DD ('99), DD ('01), DD ('05)
2004 Surveyor SV261T (UltraLite Bunkhouse Hybrid)
2006 Mercury Mountaineer V8 AWD Premier
Equal-i-zer WDH (10k), Prodigy Brake Controller