I would generally have the same answer as @5iver gave - there’s often more than one way of doing things in ZWave and I would have expected the same information on both channels.
However, manufacturers can provide whatever they like, right. So they could provide different information on different channels and this is also not uncommon. But that’s device specific - maybe the manual says something?
I gonna create a full overview of my devices, their settings and their data. I will test it again, what they are reporting. I let you know what I did find out.
I will definitely get back to this topic, but it might take some time.
Thx for your replies so far. 
Quick test with a FIBARO FGWP102, charging a bike light - where the power drawn is reduced while the battery reaches its max capacity.
channel “sensor_power” seems to get updated in real time.
channel “meter_watts” seems to lack behind or just get some updates every few minutes or so.
2021-05-25 12:41:48.119 'Z36_sensor_power' changed from 0 to 6.2
2021-05-25 12:41:49.120 'Z36_sensor_power' changed from 6.2 to 10.1
2021-05-25 13:09:53.632 'Z36_meter_watts' changed from 0 to 10.1 // 30 minute delay?
2021-05-25 13:39:53.423 'Z36_sensor_power' changed from 10.1 to 10.2
2021-05-25 13:39:53.634 'Z36_meter_watts' changed from 10.1 to 10.2
2021-05-25 13:39:53.845 'Z36_meter_kwh' changed from 12.79 to 12.8
2021-05-25 14:09:53.418 'Z36_sensor_power' changed from 10.2 to 10
2021-05-25 14:09:53.630 'Z36_meter_watts' changed from 10.2 to 10
2021-05-25 14:15:49.101 'Z36_sensor_power' changed from 10 to 8.4
2021-05-25 14:21:08.101 'Z36_sensor_power' changed from 8.4 to 7.1
2021-05-25 14:26:25.101 'Z36_sensor_power' changed from 7.1 to 6
2021-05-25 14:31:34.100 'Z36_sensor_power' changed from 6 to 5
2021-05-25 14:36:11.095 'Z36_sensor_power' changed from 5 to 4.2
2021-05-25 14:39:53.419 'Z36_sensor_power' changed from 4.2 to 3.8
2021-05-25 14:39:53.630 'Z36_meter_watts' changed from 10 to 3.8 // updated less frequently
2021-05-25 14:43:43.093 'Z36_sensor_power' changed from 3.8 to 3.2
2021-05-25 14:48:10.091 'Z36_sensor_power' changed from 3.2 to 2.7
2021-05-25 14:53:49.088 'Z36_sensor_power' changed from 2.7 to 2.2
2021-05-25 14:59:16.088 'Z36_sensor_power' changed from 2.2 to 1.8
2021-05-25 15:04:45.084 'Z36_sensor_power' changed from 1.8 to 1.5
2021-05-25 15:09:53.424 'Z36_sensor_power' changed from 1.5 to 1.3
2021-05-25 15:09:53.637 'Z36_meter_watts' changed from 3.8 to 1.3 // updated less frequently
2021-05-25 15:09:53.846 'Z36_meter_kwh' changed from 12.8 to 12.81
2021-05-25 15:14:29.077 'Z36_sensor_power' changed from 1.3 to 1.1
2021-05-25 15:15:37.075 'Z36_sensor_power' changed from 1.1 to 0.3
2021-05-25 15:15:38.076 'Z36_sensor_power' changed from 0.3 to 0
2021-05-25 15:15:50.092 'Z36WP_Switch' changed from ON to OFF
What’s weird is that in the channel descriptions, the slower meter_watts channel has the word “instantaneous” in its description.
And the sensor_power has “kWh” in its description.
These 2 minor items appear incorrect to me. sensor_power appears to me most instantaneous, and is also in Watts.
meter_kwh is the running total, which some of us reset on every 1st of the month.
HTH.
I can confirm that sensor_power delivers an immediate update of power consumption in watts, and meter_watts gives the same number, but updates very, very slowly. I have a kettle and a toaster connected to Fibaro plugs, and neither updates in the time it takes to boil water or make toast when using meter_watts. The description on sensor_power is definitely wrong.