ST in STMcube prefers FreeRTOS, Keil his RTX. Unfortunately, for example STM32F1 not exist drivers compatible with CMSIS 2.02 and so there is the question of whether to use RTX or FreeRTOS? It is one of them significantly better?
An OS allows you to respond very quickly to real time events, because an interrupt can wake a high priority task without the need to wait for your super loop to finish whatever it is doing (unless you are willing to load interrupt mode, which is usually a bad idea). So for your needs, I certainly see a benefit in using an OS. And yes, having used both above mentioned products intensively in the past (and future...) I can testify that I found FreeRTOS to be far supirior to its commercial counterpart.