According to DoD 5000.2-R, you should use either a NGS or CID to buy commercial products, but it doesn t give any guidance on when to use one or the other.
Bottom line is that if a NGS already exists that can be used as is, then use it.
If the NGS requires modification or new development, then you have to weigh the factors, but you should only pursue the NGS route if you believe it to be a good business decision. For example, paint manufacturers successfully blocked the development of a paint product NGS for 15 years. The DoD should have given up and developed a CID once it saw the united opposition by paint manufacturers and the unwillingness on the part of users to band together. On the other hand, DISC was going to develop CIDs for commercial aerospace fasteners and the industry indicated they preferred National Aerospace Standards.