Mask vs Cropping when using Ultra Key

[Nozmo]

I have always first cropped my green screen clip, then applied the ultra key.  Recently I tried using a mask instead and the result seem to be better.  Should there be a difference in the results, or was this just a random event?

[Ann Bens]

As we have no indication of what your timeline/effects controls looks like this is a wild guess:

Dont use a mask from the Crop effect but rather from Opacity.

Sometimes it matter how effects are stacked.

[Nozmo]

My typical workflow would be to bring in the green screen clip and the background at the same time.  The green screen clip is 4K and the background is 1920 x 1080.  I then would then resize the green screen clip, crop the clip, and then apply the ultra key.

What I "seemed" to notice is that if I used an opacity mask, the quaility of the key appeared better - ESPECIALLY - if I did not bring the background into the timeline until after I applied the green screen.  I tried this same sequence change with cropping and it appeared better.  So maybe, it has something to do with the timing of the stacking of clip and background rather than mask vs cropping.

Or maybe this is just a random difference?