So, I am using both and I can honestly say, how Apple has implemented this screen mirroring/using as a second screen is terrible. My system is iMac 5K late 2015 and iPad pro 9.7
With Sidecar, you cannot resize the resolution of the iPad screen but most importantly you cannot calibrate or load any other profile to correct display.
At this point, Luna Screen is more user friendly, you can use your fingers, you can calibrate the screen of iPad and also you can resize the resolution.
I am disappointed with Apple Solution as I was really looking forward to use Sidecar on my other system which is a new MacBook
With Sidecar, you cannot resize the resolution of the iPad screen but most importantly you cannot calibrate or load any other profile to correct display.
At this point, Luna Screen is more user friendly, you can use your fingers, you can calibrate the screen of iPad and also you can resize the resolution.
I am disappointed with Apple Solution as I was really looking forward to use Sidecar on my other system which is a new MacBook