Since I'm no expert in real-time threads, I was wondering which OS was doing it right, since I've read so much about OSX's RT capacities.

In OS X there are several priority bands: kernel-level, and then user-level which is subsequently split into the normal, system-high, and real-time bands.

The selectable resources include threads, timers, queues, semaphores, mutexes (mutual exclusions), and other components of the operating system.