Power Calibration - Which is more accurate?

Hi guys,
I use an Elite Zumo and their app for calibration is unusable so I normally calibrate through Zwift’s spindown calibrations. Recently, however, I invested in a Hammerhead Karoo 2 bike computer and I decided to calibrate my trainer using that. Once calibrated, I did a Zwift crit race and got a 20 watt increase on my 20min power, taking it from 260 watts to 280 watts, even though my HR remained ~185bpm (max 216).
Which method of calibration would be most accurate and is there any way to test the accuracy of a smart trainer without another power meter?

Many thanks,
Aaron Hunt