Why have two types of HMDs: those with OLED micro-displays and those with flat-panel displays?

I was recently asked why it made sense to offer HMDs based on micro displays at the same time that we offer HMDs based on flat-panel displays.

Because we can.

But seriously, here's why.

Micro-OLEDs (such as those from eMagin) have certain advantages:

• Small physical size allows for small physical size of HMD. Compare, for instance, the size of the zSight 1920 which uses micro-OLEDs with the size of the dSight which uses flat-panel displays. Both have 1920x1080 pixels per eye, but the dSight is physically larger. For applications that have space constraints or where the user needs to brings objects close to the cheek (such as a gun), a small HMD has a big advantage.
• Micro-OLEDs current offer higher refresh rate: 85 Hz as opposed to typically 60 Hz for flat-panels. 
• Most available flat-panel displays use some version of LCD technology. OLEDs offer superior response time and contrast. However, several vendors have announced (or are already selling) OLED flat-panel displays.
• If you care about pixel density, it is easier to design an optical system that would provide very high pixel density - even eye-limiting resolution - using a small micro-display. High pixel density implies lower field of view for the same number of pixels. You would care about high pixel density if you need to see every bit of detail in your image, or need to detect virtual objects at far distances, such as in military training.

Flat-panel displays have different advantages:

• Their cost is much lower since they are key components to cell phones.
• Larger supplier diversity.
• Much easier to create very wide field-of-view systems than with the micro-OLEDs. If you care about immersion, you can usually get more immersion with flat panels. Of course, wider field of view implies lower pixel density.
• Resolutions are rapidly increasing. 1920x1080 seems to be the current standard for high-end phones but this will soon be displaced by 2560x1440 or other high resolutions.

Ultimately, there would be many more HMDs that are based on flat-panels, but there are unique professional applications that would continue to prefer OLED micro-displays.