antenna sonnet
16 patches should be no problem provided you can keep the subsection count down under about 20,000. On a 4 GHz PC this will allow computation in about 20-30 minutes per freq and will fill up about 2 GBytes of RAM. You might want to invoke the Memory Saver option (switches to single precision, cuts memory requirements in half). You can also invoke the Memory/Speed Tradeoff slider bar to increase subsection size with (usually) a very small trade off in accuracy. Both of these options are under Analysis -> Setup. These options are described in detail in the manual.
With 15 patches, you should keep the cell size on the large side and not have extremely intricate patch geometries. If there is some 3-D aspect to your patch, esp. with respect to the substrate, CST would indeed probably be the best tool to use. All depends on your geometry.