End-to-End is better outside Gmail. Because it is an extension, you can encrypt/decrypt any message in any webpage such as: web forums, other web mail providers (yes even Yahoo Mail, Outlook.com, etc), or your custom internal SquirrelMail or Outlook Web Access instance, etc.
Its open source can be reviewed by third parties, it can be built and installed locally, with no dependency or trust placed in Google's online services. Heck, it can even be (in theory - I never tried) installed on Chromium if you are paranoid and don't trust the few non-open source parts of Chrome.
Google has made this a fucking nightmare. Every time you start Chrome, you get an annoying popup "do you really want to enable local extensions blah blah"
Its open source can be reviewed by third parties, it can be built and installed locally, with no dependency or trust placed in Google's online services. Heck, it can even be (in theory - I never tried) installed on Chromium if you are paranoid and don't trust the few non-open source parts of Chrome.