It really depends on what your individual goals & needs are. In this case a 'mirror' type add-on like this will retrieve hotlinked images embedded with the img
BB code and convert them to actual post attachments whereas the image proxy will leave them as embedded images.
What I perfectly understand, is the difference as explained by you here. But I need to understand a little more, to actually be able to decide, what meets my goals and needs. I do have Mirror Images licensed...
My main point is GDPR - I don't want, that the members ever open an image, that is pulled from another server, transmitting their IP etc. That should be the case with the addon, though it does not include profile posts.
The proxy on the other hand, has settings for the time images are cached locally. What happens, if the cache is emptied - will the image be renewed in the background, or when the next member opens it? Will really ALL images be cached, even very small ones (like smilies from smilie sites some people love to use)?
Also, I show some attachment stats, so users are vigilant about the space images may use up. I suppose, as with the proxy the images are not attachments, they would not show up there, while still using server space in the proxy cache?