If you are to go by the introduction software, i.e. which OS is available for new products first, then the answer that ST presents is AZURE.
However, it may not be that simple.
If you ever do C++ projects (and there are some applications which make much sense in C++), neither OS is perfect. However, I have had more difficulties with AZURE unable to deal with elements part of a C++ class than FreeRTOS.
The two OS systems are not fully interchangeable, however. Specifically, the queue system in FreeRTOS allows for unlimited (reasonably so), queue entries, while the queue system in AZURE is limited to 64 bytes(or 32 bit words, forget which). AZURE wants to have queue entries passed by reference, which can make life difficult. Larger entries need a separate mechanism to handle them, and you have to consider storage and for how long that storage will be valid.
I also had a lot of difficulty working with some AZURE members from inside a C++ class.
FreeRTOS doesn't like having task code owned by a class, therefore the FreeRTOS tasks have to have workarounds to access class variables. They deal with classes well enough, but the task has to be a separate piece of C++ code and not in a class.
If you don't want to, or never will, use C++, then it's going to be easier.
If all your queues can be short, then AZURE may work better.
You can always bolt on FreeRTOS even if it's not directly supported, but that tends to be fussy.
I tried taking an entire (large) project from FreeRTOS to AZURE, using #ifdef and rewriting code where needed. It did not work well for me because of the items above. AZURE has better graphics diagnostic tools included, although you could possibly buy or write something to do tracing and task monitoring. That depends on whether or not you're working with a company ($$$) or not (no $$$).
Another possibility might be embOS, which is available as a package for ST Micro chips. I've looked at it, but not explored it thoroughly. It may have the same problem with C++ and scope of class variables, but I don't know.
So which? I'm back to FreeRTOS which matches my needs more closely. I've learned to work around some limitations in FreeRTOS, where AZURE did not work well with my code.
Hope it helps.
