Having used both for real, complete projects, I would say they are both robust and well-documented. However, I found less rapid response on the Mason help list, but the Embperl list was much faster to help and I got more help.

On top of that, I have at times pursued the commercial support options for Embperl and other HTTP-related technologies and I don't know of such features for Mason.

Finally, Embperl is supposed to be moving away from HTML-dependency and will support XML generation as well. And Embperl works in 3 modes: offline, CGI, mod_perl. I believe Mason operates in the latter two, but certainly not the first.

That being said, XML seems to be all the rage and the two web application environments for Perl which are XML-oriented from the bottom up are AxKit and Apache::PageKit and I am doing a webapp using AxKit on the Perlmonk's user site right now after finding CGI::MxScreen to be a bit limited in its paradigm.