For VR Porn it really depends. Normaly the 128gb version is enough even for storing a few videos on there.
But if you also plan to watch regular 3D movies in bed or plan to play games without streaming them from your pc for example i think the 512gb is the best choice.
There is always the option to set your pc or home server up as mediaserver - many vr players support DLNA for example which lets you access the files on those pcs/servers for every folder you link into your media library - so then basicaly all you do is stream your files over your local wlan.
