How to choose OS?

Criteria of choosing OS for my embedded system:

• Cost (Free - commercial)
• Requirements:
•  The used HW shall support the OS
• Some OSes support only 32 bits processors
• Some OSes requires the existence of MMU (Memory Management unit)
• Portability
• Can I migrate my OS to another HW?
• Can I migrate my app to other OS? 
• Using other words, Are the OS APIs standard APIs? If so, the mogration from OS to othe OS will be easy
• FootPrint
• How much memory is required to:
• store the OS
• run the OS
• Real Time
• Fast enough
• Predictability
• Configurable
• Customizable
• Support
• Open Source
• The code is visible, I can change the behavior of OS, customize it, debug in it.
• Open source != Free
  But, Somehow, they are related !