Procyon:
.4 minimum ohms (can sub ohm, but not super low)
50W device.
18650 ONLY
Updated switches/bottom cap/top cap to allow the higher voltages/wattages.
P3:
.5 minimum ohms (can subohm, but barely)
20w device
can support 18350/18500/18650 batteries
Real world use: I like a .7 ohm build on my regular coils. If I use the P3, it can give a maximum of 3.7 volts.. procyon can go up to 5.9 volts..... I usually vape a .7 ohm build at 30-35 watts, so the P3 wouldn't cut it for me. In short, I believe the Procyon is just a newer and more powerfull version. If you vape 20w or below, then either will be fine, with the P3 having the option to use smaller batteries.