Actually, the Primus IQ is fairly new. The P2 is the original Prodigy and the P3 is the latest and greatest.
The Primus IQ uses the primary sensor technology from Prodigy P2 according to Cequent. They are both proportional and detect how hard you are braking.
I think they are both good controllers, with the Primus being marketed as "economical" and the P2 as more flexible.
The P2 can be mounted many different ways - straight up, straight down or anywhere in between. The Primus is limited to a range of 0 degrees to 70 degrees nose up.
There are differences on the display and control buttons.
The P2 uses a rotary lever on the underside for a manual over ride whereas the Primus uses the older slide design. There are some differences in the buttons. I think both of these differences are personal preferences.
The P2 can be used with electric over hydraulic trailer brake systems.
I believe that the P2 has more diagnostic abilities to help you troubleshoot problems with the brakes and brake wiring. You need to confirm that, tho.
One you didn't mention but is worth considering is the P3. Its like the P2 but costs more and has many more diagnostics (i.e., its fun to play with