FitBit vs Garmin

Good Morning :)
I find FitBit and Garmin are showing different results from the exactly same exercise. Fx. yesterday I took them both for a test and went on an hours bike ride with both on. The calories burned on FitBit was 345 and on Garmin 674. I did have the FitBit on my wrist and the Garmin Heart rate monitor on my chest. But the difference is too big to be realistic.
Does anyone have experience of which is most accurate? Is the wrist-reading that much out? Or, is Garmin very generous?
Looking forward hear your opinion,
Thanks.