I prefer Maildir but mostly because it is trivial to manipulate because it is so simple. Our design back then was based on the assumption that we could blow away all the cache files at any time, and it could all be trivially rebuilt from just the raw messages.
I still have a soft spot for flat files when possible for that reason.
Also for our usage we saw a relational DB as a scalability hassle, as we needed to be prepared to scale this to many backends, and dealing with an RDBS when each set of objects was specific to a user was not very appealing. Today the challenge in that is much smaller due to faster hardware, and there are far more options in terms of database solutions.
But at the same time there's just not that much to gain.
I still have a soft spot for flat files when possible for that reason.
Also for our usage we saw a relational DB as a scalability hassle, as we needed to be prepared to scale this to many backends, and dealing with an RDBS when each set of objects was specific to a user was not very appealing. Today the challenge in that is much smaller due to faster hardware, and there are far more options in terms of database solutions.
But at the same time there's just not that much to gain.