I had a P2 but was planning to get a P3, until the new truck with it's built in controller appeared.
AFAIK, the P3 has a few more bells and whistles than the P2 but basically operates the same.
The P3's most important bell/whistle to me was the ability to check the current output of the P3. You can't do this with the P2. It's a bit convoluted to do it on the P3 (under "Troubleshooting" IIRC), but it does allow you to test your brake system and ensure all brakes are operating as expected.
A couple of us have had broken brake wires and only one out of two brakes working without realizing it. A routine check of the current would have diagnosed that. Monitoring the voltage (the normal display for the P2 and P3) does not tell you that.
Unfortunately, the built-in brake controller in my new truck does not provide current monitoring (although it has some other really nice features). But I've purchased a tiny digital ammeter and once I get up the courage, it's going in, somehow.
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