We Rubyist have rubygems.org. This is awesome, but this is not api friendly(e.g. CORS), and this doesn't have enough meta data. This is why api.metarubygems.org comes.
We metarubygems.org team are inspired cpan(Perl) and npm(Node.js), cpan have compare view between revisions.
We build compare view between revisions. http://api.metarubygems.org/v1/gems/activerecord/compare/4.1.5...4.1.8