Differences between SSRN, arXiv, academia.edu, NBER working papers, et al
As a general rule, you should write in some medium if you read that medium. So you should send your work to a depository if you follow that depository. (See this advice on cross-listing in arXiv.)
If you do not follow any, start following them. If you follow several, choose the one that you prefer as a reader.