They are very different beasts. Arxiv (and most pre-print repos) accept submissions (with filters, like requiring academic affiliation or vouching, and requirin

They are very different beasts. Arxiv (and most pre-print repos) accept submissions (with filters, like requiring academic affiliation or vouching, and requiring reasonably-formatted metadata), have a moderator do a skim-level review of the work, and then post it. CiteseerX is an automated crawler, like Google Scholar, which finds PDFs on author homepages and extracts metadata from the PDF. There is no human review or cleanup process, and there can be a long delay before content gets discovered.