Fortunately, real-time vendors and open source collections offer plenty of off-the-shelf, ready-to-run software packages to fit most embedded configurations. The challenge will be to analyze your requirements to match an embedded computer operating system to the application with ample room for growth while minimizing the cost and development effort. Although a commercial OS can be expensive, cost savings is an important reason to purchase an off-the-shelf product. If you can purchase and therefore eliminate the coding, debug, and documentation of the most complicated portion of the software, you should give it careful consideration. Vendors promote product technical support as a major benefit of a commercial OS. They are able to provide continuous embedded computer support for the operating system portion of the software by spreading the cost over all customers.
refer to: http://embedded-computing.com/articles/choose-right-embedded-operating-system/