I probably know the answer but just in case.

If you stumble with the same art from an artist on two places:
- On Twitter that's uncensored but it's with a lower resolution or size
- On Pixiv but censored

What version shall need priority or be uploaded? Or can both be on e621 at the same time?

To my knowledge both can be posted. Most people will prefer the uncensored naturally.

if there are censored and uncensored versions of effectively the same artwork: than both should be uploaded, regardless of image quality so long as it's the best version available

of course in a perfect world either the artist hands over a lossless version of a piece when asked for permission by an uploader or they'd just upload somewhere that hosts lossless images in the first place like inkbunny or itaku

Though they seem to choose "old one on the site" if the only matter is slight quality difference. It doesn't have to be the best version.

As far as I know, if the images are otherwise identical (i.e. they're the same size, and both lossy JPGs with the same amount of artifacting), just that the Pixiv version is censored, just the uncensored one should be posted. However, if the censored version is larger with the same or better quality (or the same size with better quality; either lossless PNG or less artifacting), they can both be posted.

If the quality between the censored and uncensored art is equal other than the censorship, then the censored version is inferior and will be deleted when flagged.
If the uncensored version has superior quality as well, then the censored version will be deleted.
If the censored version has superior quality aside from the censorship (PNG, higher resolution, better compression), then that version usually stays up until an equivalent or superior uncensored version can be sourced.
(I'll add that all this only applies to official uncensored versions. Third party decensor edits are not "superior.")

Understand that censorship is literally a reduction in quality that counts against that art. Even so, that reduction in quality is usually highly localized, so otherwise superior censored versions are superior in the rest of the art. As I have been told, "quality" here is synonymous for (more/accurate) "details," which can mean a variety of things that I'm not going to bog down this post with. But I will say that destroying a section of art with censorship is a pretty significant loss of quality/detail.

Some posts are well-received in spite of their censorship or low quality. Others don't overcome their flaws. The only time I spend on censored art anymore is hunting for replacements to purge them from our site.

abadbird said:
If the quality between the censored and uncensored art is equal other than the censorship, then the censored version is inferior and will be deleted when flagged.
If the uncensored version has superior quality as well, then the censored version will be deleted.
If the censored version has superior quality aside from the censorship (PNG, higher resolution, better compression), then that version usually stays up until an equivalent or superior uncensored version can be sourced.
(I'll add that all this only applies to official uncensored versions. Third party decensor edits are not "superior.")

Understand that censorship is literally a reduction in quality that counts against that art. Even so, that reduction in quality is usually highly localized, so otherwise superior censored versions are superior in the rest of the art. As I have been told, "quality" here is synonymous for (more/accurate) "details," which can mean a variety of things that I'm not going to bog down this post with. But I will say that destroying a section of art with censorship is a pretty significant loss of quality/detail.

Some posts are well-received in spite of their censorship or low quality. Others don't overcome their flaws. The only time I spend on censored art anymore is hunting for replacements to purge them from our site.

That's wrong. The censored version counts as a variant of an image and can be safely posted for the same reason I can post a dirty feet version and a clean feet version of a foot fetish draw. The only time it may get deleted is if the censored image is worse or equal quality to the incensored image.

wolfmanfur said:
The only time it may get deleted is if the censored image is worse or equal quality to the incensored image.

That's what he said:

If the quality between the censored and uncensored art is equal other than the censorship, then the censored version is inferior and will be deleted when flagged.
[...]
If the censored version has superior quality aside from the censorship (PNG, higher resolution, better compression), then that version usually stays up

I've had censored posts replaced with uncensored ones when the uncensored one is of equal or better quality and size.

notknow said:
I probably know the answer but just in case.

If you stumble with the same art from an artist on two places:
- On Twitter that's uncensored but it's with a lower resolution or size
- On Pixiv but censored

What version shall need priority or be uploaded? Or can both be on e621 at the same time?

It's a long shot but sometimes you can ask the artist themselves if they intended to upload a low res (web res) image to twitter, and if they could make the higher res one available? I've told people on FA before about the reupload trick because of the annoying resize stuff and I know they'd rather get around that if they can help it.

Very rarely, the censored version is actually done for humor. I'd assume those are allowed variations. But black bars and light sabers and blurry windows seem to be the most common. Part of the logic for leaving the censored version is if it is not certain if it's official or not. It's not always easy to find that out.

One artist I uploaded art from, has their own site with higher-resolution versions publicly available and not behind a paywall. It's on backlog of things to do.