comes and dux

[D B Campbell]

Beware, duces and comites are NOT ranks, they are titles. But in general (no pun intended) one can say that a comes was in command of more important armies and regions than a dux.

I think you're right, Robert: comites were not necessarily military, and the title did not denote a particular rank. Some comites were given military functions (the comes maritimi tractus of Amm. Marc. 27.8 springs to mind), while others were not.

However, in general, I think it's true to say that a dux was in command of a frontier region, and in that respect it can be considered as a rank (imho).