[Dovecot] mbox vs maildir

I've had clients 'request' nested folders, and it would seem that maildir is designed with that ability while with mbox it is difficult and.or impossible to implement (nested can be achieved; but not nested AND populated in each nest level).

My question is, is one format 'better' than the other? It would take a fair bit of time to convert my system to maildir and I would want to feel comfortable that this would be a true 'upgrade' in abilities, rather than simply a change to accommodate nested folders.

Thanks,