When you are deciding between ordering a managed VPS server or an unmanaged VPS server. You should be asking yourself if you are technically capable of managing the server by yourself or not. It is that simple.
The costs of an unmanaged VPS server will typically be a lot lower than compared to a managed VPS server. With a managed VPS server the web hosting provider is responsible for the making sure the VPS software is up to date and has to provide technical support to you. An unmanaged VPS on the other hand comes with barebones technical support. The only technical support you should expect to receive if you order a unmanaged VPS server is network connectivity and reloading the OS related issues. Everything else will probably not be provided by your web host.
If you are technically capable of managing your own web server and want to then going with an unmanaged VPS maybe optimal. You can manage the VPS server by yourself and pay a lower cost than compared to getting a managed VPS server. You would be responsible for all the software and making sure everything works as you want them to.
If you are technically capable but do not want to spend time on managing a VPS server or assume the responsibilities of keeping the software up to date everyday. You may want to order a managed VPS instead. With a managed VPS you get technical support from the web host, a control panel to manage the VPS server is typically included, the software and responsibilities of making sure the VPS server is operating smoothly rests with your web host and not you.
If you do not know anything about the inner workings of the VPS server or server software technologies. It would be wise to go with a managed VPS server instead of a unmanaged VPS server until you get more technical experience.