You're 11 times more likely to break a nail when typing the latter. Other than that, they're identical. c is simply a shortcut for cpu-shares
To prioritize the OS over a container, you're best bet is to explore https://docs.docker.com/engine/reference/run/#cpu-share-constraint and below
You can set the priorities to anything you want. Every container has a default setting of 1024. The lower you go (down to 2), the lower the priority of the container is relative to other containers. The higher you go (up to 2048) the higher the priority of the container is. It's easiest to visualize it only considering 3 containers running on a single core CPU.