We (1293) used belts in 2019 because we used the AM14U4 chassis, and there is no upside to trying to re-engineer it.
We used chain in 2018 because we could not hold suitable tolerances to make belts work in our (bad) custom frame.
We (5402) used belts in 2017, because AM14U3.
We (4901) used chain in 2016, because we designed a custom chain-in-tube drivetrain to let the chain hide inside the tube for protection. I was happy with this, but would need to have that kind of milling resource again to repeat it.
We (Ri3D Team Cockamamie) used belts in 2016 and 2018, both times because AM14U3.