TLDR: Zero-K and BAR are based on the same FOSS engine. Zero-K is less micro with better unit AI. BAR delivers a more vanilla and micro intensive Total Annihilation experience.
Supreme Commander is proprietary closed source and has some different mechanics. Larger but worse maps.
Found the answers in the Bar FAQ: https://beyondallreason.info/faq/what-are-the-differences-between-bar-and-zerok
TLDR: Zero-K and BAR are based on the same FOSS engine. Zero-K is less micro with better unit AI. BAR delivers a more vanilla and micro intensive Total Annihilation experience.
Supreme Commander is proprietary closed source and has some different mechanics. Larger but worse maps.