To compare the two:
Zero-K allows 16 vs 16 battles (and it works!), each player controls a relatively smaller number of units. In Beyond All Reason, there are 8 v 8 battles and each player controls a larger number of units, maybe 200 or so at the peak of battle. (There are some strong, but predictable AIs for both games.)
Zero-K has more units that are less realistic and have more variety. BAR has more realistic units and less variety. BAR has tanks, Zero-K has flying tanks that can also turn invisible (this is a made up example, but illustrates the idea).
Zero-K has a limited economy, you have to control land to to generate income, and fighting has more influence on who wins. In BAR, the economy is more exponential, and at high tech levels controlling land doesn't matter, you can generate a huge income in a tiny corner of the map and win because your economy is exponentially stronger than anything your opponents have.
Is Zero-k still popular now that its cousin https://www.beyondallreason.info has matured ?