I've never played much Zero-K, but from I understand the eco in BAR is truely exponential, after you get to the top of the tech tree and have a metal maker economy. It's an interesting design, since late game human becomes the bottleneck in expanding the eco, since exponential growth also means you have to increase your attention in managing it.

Zero-K from what I know has overdrive fusions, which isn't really exponential in the same sense.

In the end, both are great games.