BAR is more similar to the original Total Annihilation (and successors like Supreme Commander). It's a lot of similar units (e.g. tanks) with subtle variations, multiple tiers, with a focus on out-numbering your opponent.

Zero-K went a different route. They simplified the economy (metal cost = energy cost) but added a whole lot of mechanics like overdrive grids and made every unit unique. There is a lot of variety with mind-control, jumpjets, shields, unit-throwers, self-replicators, teleports, terraforming, etc.

Zero-K has been on Steam for a few years, I think BAR is working towards it. They are both good worthwhile games, they share some common elements, but they are both worth trying in their own right.