Given the size and wealth of Volantis (its satellite cities are each larger than King’s Landing) and the size and wealth of Braavos, which would you say is the stronger? In a protracted war, which would hold the advantage?

I think they’re about the same, but Volantis is the waning and Braavos the waxing power. In a protracted war, I’d give Braavos the advantage because they’re better able to finance their war effort and to recover from naval losses thanks to their highly efficient Arsenal.