A self-managed VPS is best suited to those who have experience of running a server - when you have a unmanaged server you will be responsible for managing everything from the set-up right through to the daily monitoring and maintenanace. This also means that you get to set everything up as you please, choosing your preferred operating system and Control Panel.
If you do not have the experience running a server and want an expert to take care of the initial set up, health monitoring and backups then a managed solution would be best for you.