A blog post from Hugo @ TrueRC regarding the differences between his OCP and Singularity range of antennas.
The two most popular antennas from TrueRC for 5.8G side by side, with some of the main differences between them.
Dimensions: The OCP is 16.5mm diameter whereas the Singularity is only 11.8mm.
Weight: The OCP head is 1.31g whereas the Singularity is only 0.56g.
Performance: The singularity has better pattern fidelity, less disturbed by proximity of objects or a very long cable. It also has slightly higher gain and thus the edge over the OCP.
Costs: The OCP is cheaper.