They shoukd all be weighed on the same day with the same scale.
Chain drive is ISO 406 wheel. Belt drive is ISO 451. Some part of the difference.

If you want a MU Uno and can find one, I would buy it. Had one and have sold several. Personally though I think the Vitesse D3 is the cool bike for the $. Almost gone now. 3 speed internal main difference to the rider (can't easily make belt drive though but could wrap the chain ala Speed Uno).