To me, either tag is like marking a question "code". It adds no useful information -- especially for categories like "Greasemonkey" which imply a user-generated script, anyway.
I respectfully request that these two tags be merged or eliminated.
I manually merged/deleted the userscript tag from all questions.
Please vote to approve userscript as a synonym for userscripts at this link. Done, thanks!
Now, holding out hope for a purge or a justification for the "userscripts" tag...