The major difference between torrents and usenet, torrents is shared over several hosts "PC's that downloaded the content and is sometimes subjected to the tracker to keep the file seeding for 48+ based on their rules, but in a nutshell it is a cheap, almost non costing way of getting information, on the other side, usenet is based on news servers using articles to make up the content, thus it has a retention ratio of 1800+ days and it is not relying on other devices having the same information as all the data is stored locally on their news servers, but, with usenet there is costs involved, as InvaderGizmo also explained above, you will need to pay for an indexer and a usenet provider, some usenet providers only keep data 1000 days, others 1800+ days, it is all based on your needs.

So for free -> Torrents
If you want old / rare -> Usenet

If you need more info, please let me know.

Kind Regards,
SpiderCrawler