Open source products are generalized in order to fit the widest array of users and can force designers to modify the embedded computer hardware configuration, resulting in higher recurring cost for the embedded device. 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 support for the embedded computer 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/