I don't have any experience with the P3, but I've got the Tekonsha Prodigy, and it was worth every penny.
When pulling a smaller trailer or my big horsetrialer empty, you can push the button for the first setting. Meaning less brake pressure to the trailer. then when the trailer is loaded you push the button again and it addes a little more pressure to them.
So there is no re-adjusting or hassle. Set the controller once for the heaviest load you'll pull, then forget it.