Power meter vs. smart trainer power

I have a Saris M3 smart trainer and really enjoy it. I recently got power meter cranks, Qwark, and found the power in Zwift is reading about 15-20 watts less on the trainer vs. the cranks.

What is the better or expected power measurement for Zwift? Is it best to use what I am putting out at the pedals or what is being put out at my tire?

Thanks Zwift community! This place rocks!