Hello, I'm going to buy Quest 3 and I'm wondering if 128 GB of memory is suitable or is it better to pay extra and buy 512 GB?

If you stream directly or download to a PC or laptop you don't even need storage

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.

Stream VR videos from SLR website or download and stream from your PC/network/NAS

For games you may need storage, have a look at the size of them on average and what you might want to install