I am confused about which of the Sonoff Zigbee USB P or E I should chose from and which one of the two is better to work with the IKEA TRÅDFRI. I have read lots of things about the two Sonoff. -E is the most modern but may require more maintenance in therm of firmware flashing. Unless mistaken the -E version does not support Z2MQTT. I wanted to know if they both work with the IKEA TRÅDFRI and your recommendations. HA is installed in a Raspberry Pi3. Thanks.
All you need to check is compatibility between the Sonoff and ZHA/Z2M, then between the Tradfri and ZHA/Z2M.
It is the zigbee software which handles compatibility between devices, not the hardware itself.
Side note - your Pi3 might not have enough RAM to run a zigbee integration on top of your existing install. Keep an eye out on the RAM consumption just in case you run into issues.
As far as I understand both version P and E should work with the Ikea Tradfri and Sonoff. I would like to stay on the safe side with the P (although it looks like the E version is more stable now and can be flashed easly). In both cases I need to flash as router the new Sonoff prior integration into HA. Correct? I will check about the RPi3 ram. Thanks for the advice.