For AR it depends on if you can see through the screen like Google glass or if it’s a VR headset with external cameras. With pass through you can’t change the facial length of the outside world, but you want to change it on your overlays.

In terms of VR it depends on how well you can pull off the blur based on eye tracking vs how much control you have over focal lengths. A perfect version of either is probably equivalent, but doing both reasonably well is probably easier than taking just one close to perfection.