One thing that we should have in consideration is that
jsMath has a weight 10MB
MathJax has a weight 33MB
Because this, it is hard to think in a off-line version of documentation for each package.
Maybe, because that jsMath is a older version can be cleaned (discarding some fonts).
By other side, as says Oliver, MathJax shows more complete, supporting MathML.
Bun I don't see a poor quality without MathML. What parts are failing in MathJax?
Fernando