Picture courtesy of Birmingham Sealife Centre

Head Mounted Displays (HMDs) come in a variety of sizes, weights, resolutions and Field of Views (FOVs).  Picking the right headset for you will depend upon what you are hoping to achieve.

What FOV do I require?
This is depends on how immersive your application needs to be.  In general, a 35-50 degree FOV is considered good enough.  However, a 100 degree plus FOV can be required when an application requires a great deal of peripheral information, as with driving simulators, for example.

What resolution do I require?
How much detail is required will determine the resolution quality.  A resolution of 1024 x 768 will suit many applications, but the latest HMDs are can now give a resolution of 1280 x 1024, so if detail is vital, these are the type to choose.