If you are familiar with hosting control panels, shell commands, and you have basic server administration and troubleshooting skills, then unmanaged VPS is the way to go. Note that while you purchase self managed VPS, you should be capable of managing applications, software, services installed on your VPS. Also, you must know installation/uninstallations, resource monitoring, server performance tweaking and troubleshooting of any technical issues.
Having your own guaranteed CPU and RAM gives you a great deal of autonomy. You don’t have to share them with any other accounts, so no matter what anyone else does they will never slow you down. Regular shared hosting is okay if your website traffic doesn’t change much, but if there’s a chance it might need more horsepower then VPS is far more flexible. And from a security point of view, the fact that VPS hosting accounts are cleverly separated on the same server makes them much more robust than a shared hosting solution.
A VPS doesn’t just have more RAM, disk space, and a proprietary share of CPU than a shared account. Depending on the provider, some VPS plans offer burstable memory, which is a pool of RAM set aside for extraordinary events. This is the stuff that can help counter the so-called “Digg-effect,” that much-whispered about occurrence that’s the simultaneous hope and fear of everyone who runs a web site. When you have an unexpected high traffic event, burstable memory will call on a pool of reserved, shared memory to satisfy the needs of temporary high traffic. This is not available on shared servers and, while the necessary memory is available on a dedicated server, your site might not get the kind of traffic on a daily basis to justify the expense of a dedicated server. Again, not all VPS plans have burstable memory, so ask your provider if their VPS plans do.