Marc Bernatchez, from VResources, has an interesting article about comparing HMDs. He compares angular resolution, field of view, stereo overlapping, and the relevance of all these factors when compared to human visual abilities.
For example, here’s the summary of the angular resolution analysis :
There’s also an extensive comparison at Stereo 3d by Christoph Bungert : http://www.stereo3d.com/hmd.htm