Repository or university working paper series
People actively follow arXiv. They subscribe to the RSS feeds, mailing lists, Twitter bots, etc. that inform them about recent relevant preprints that appear on arXiv. Google Scholar finds new papers on arXiv quickly, and this way the authors will get quick notifications when something that cites their work appears on arXiv. ArXiv papers are also indexed in other widely-used databases (e.g. DBLP in computer science).

I don't see any reason to use a local working paper series. In terms of visibility, it doesn't sound much better than e.g. simply hiding the paper somewhere on your personal home page and hoping that Google will eventually index it and someone will accidentally find it that way.