Is the move between positions live?
If live - probably best to use keystone on the layer...

But you may have to use some mix keystone to line it all up first anyway

Difficult to answer without seeing it...

I'd usually suggest using mix keystone for 'fixed' screens / positions - then anything you send to the mix will be correct...

Then use layer keystone if the image has to move live from one place to another...

(but it is also possible to control mix keystone via dmx... and recall different keystone mix setups...)

Suggest doing whatever seems simpler...
Try with the mix first - as the images are blended, you may have to anyway.

S