I will be receiving both.
Essentially the 5k doesn't need to super sample as much to produce a quality picture.
Where as the 8k needs more base super sampling to have a good looking upscaled image.
The 8k runs like a rollercoaster.
Vr ss 6k downsampled to 5k process complete nice image (5k headset)
8k may need to render at 6.5k to 8k then downsample to 5k (nicer looking 5k image) upscaled to 8k (image quality on par or bit better than 5k native panel).
Think of a reg 2d game on a 1080p monitor using DSR/VSR or in game SS renders higher for better looking 1080p image without need of Antialiasing.