We are digital librarians. Among us are represented the various reasons to keep data -- legal requirements, competitive requirements, uncertainty of permanence of cloud services, distaste for transmitting your data externally (e.g. government or corporate espionage), cultural and familial archivists, internet collapse preppers, and people who do it themselves so they're sure it's done right. Everyone has their reasons for curating the data they have decided to keep (either forever or For A Damn Long Time (tm) ). Along the way we have sought out like-minded individuals to exchange strategies, war stories, and cautionary tales of failures.
Depends on the usecase. For me I just needed a format to store lots of small files in, so I chose zip because extracting the individual files is much faster. 7zip encodes in large blocks so you needed to extract a block of lets say 512 MB to access a 42 KB file.
But just like what the other guy said, if you need encryption or more options 7z might be your best bet. I don't see the file types going obscure in 20 years.
.rar with 10% recovery record
They'll both be readable for the foreseeable future. 7z has the advantage of better compression, but other than that they are pretty much the same.