Recently, Kavitha with BCS, has put together a great comparison of the USFM Grammar processor and the USFM-js processor.
The great part about this is that she does an excellent job of showing a snippet by snippet output illustrating key differences based on various USFM marker types.
See the full details below.
https://github.com/Bridgeconn/usfm-grammar/blob/master/comparison.md