Repository or university working paper series I am from the field of economics and preprints are ok with econ journals. I just cannot understand what is the difference between repository (like ArXiv) and publishing results in some working paper series? 

I read somewhere that repository assigns DOI and working paper does not. Also, it is possible to update the repository's version, and I am not sure about working papers. From another point of view, working papers might be registered to more databases and make one's research more visible to the public....

So what are the main pros/cons in publishing preliminary results in a repository and in working papers (or discussion papers series??)? What are the differences?
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.