Hi
I am running a TACX T2060 and also have a Garmin Vector 2. In the pairing screen I can see my smart trainer as well as the Garmin and the power outputs. There is a power discrepancy of about 20 Watts between the two. It appears to me that the TACX is overstating my wattage but either way I am now unsure as to which device is the most accurate and which one to use. Any reason for this? Is there a way to calibrate the trainer to Zwift?
They messure power re power at different places, so that could cause the power discrepancy you are seeing. However, I would think the pedals would be higher since they are closer to the power source (your legs). Saying that I would most definitely say that the pedals are far more accurate than the trainer. I use my 4iiii when using Zwift even though my trainer broadcasts power, because they are far more accurate and stay accurate over time.
As of right now Tacx trainers are not available to calibrate within Zwift, so you would need to use the Tacx app to calibrate it.
I will stick with the Garmin output (which means I get dropped ha ha) I have also found a TACX app that will allow me to calibrate the trainer so I will give that a try.
Thanks so much for your help 
No use calibrating the tacx until the tyres have been thoroughly warmed up. 10 mins at least. Otherwise the smart trainer can be as much as 80 watts over your real FTP.