PNG: Intact prompt metadata, but big file size which will impact storage, as well as bandwidth for both server and client.
JPG: Negligible quality loss due to compression, much smaller file size with regards to storage, faster to serve and download.. but prompts would have to be posted as a comment if they were to be shared.
Edit: Since I am paranoid that e.g. Windows account/user info data may be stored in the images, combined with noticing there's a tag for [prompt in description](/tags/prompt+in+description), *and* it will save on space *and* be more speedy, I will opt for jpg personally, unless told otherwise.