Windows vs Linux
To start with, you need to know which type of VPS environment you will run: Windows or Linux. Linux is a different ballgame than Windows, but certainly has its perks. Regardless of wants and comfort levels, if your site is running on ASP or ASP.net, you will need to use a Windows environment.
Managed vs Unmanaged
In shared hosting, you don’t get root access to the server and hence the question of managing the server does not arise.
But when it comes to VPS hosting, the whole virtual server is handed over to you. So, there needs to be somebody to look after it and monitor its performance. If this is taken care of by your VPS provider, then it is called managed VPS, whereas in unmanaged VPS you need to take the responsibility of your server all by yourself.
Unmanaged hosting requires you to monitor the performance and keep the server running in good health. If your server software happens to crash, or some security issue manages to creep in, you are the one to fix it being the only administrator of your VPS.
So, if you are a seasoned geek, who knows the ins and outs of server management and are familiar with stuff like shutting down, repairing, restarting, rebooting the server, then you are a good candidate for unmanaged hosting. Else, you should choose to pay a little more and go with managed VPS.
The extent to which a VPS is managed varies from vendor to vendor and even with different plans with the same vendor. So, you should keep this in mind while comparing different VPS plans or vendors before you go ahead with finalizing one for you.
Most VPS hosting providers offer managed VPS hosting by default, but unmanaged does allow for some additional freedom – provided, of course, that you know how to and are comfortable managing and configuring everything by yourself.
Redundancy and Scalability
Redundancy basically refers to having a backup resource in place, especially at the data center. If the regular power supply fails, generators and UPS systems should be there; if the services of the ISP are interrupted, some other alternate arrangement should be there, if one server is overloaded, another standby server should be there; so on and so forth. Scalability, on the other hand, indicates the ability to handle sudden, occasional increased loads on the server, usually by making use of the redundant resources in the system. Both of these combined together translates into higher uptime and consistent performance.