Webstorm or Dart Editor. Which one is more mature
WebStorm team was working on Dart support for over a year now, getting regular feedback and assistance from the Dart team. So I believe that Dart support in WebStorm is quite mature.
The overall quality of code completion, navigation, code inspections (the same analyzer is used as in Dart editor) and quick fixes offered by WebStorm might make the difference. WebStorm has built-in debugger for Dart web and command line apps.
Moreover, WebStorm adds value by providing support for HTML and CSS (LESS, Sass, whatever you use) in your Dart project, while Dart editor doesn't. Have a look at the recent discussion in Dartisans Google+ group.
from the official site
As of 1.11, Dart Editor is obsolete and no longer supported. We recommend using DartPad to play with Dart and WebStorm to develop with Dart. For more information, see the announcement.
Also from The present and future of editors and IDEs for Dart
Another option is the Eclipse plugin for Dart, which we continue to support. The Eclipse plugin, which currently shares most of its code with Dart Editor, is powered by the Dart Analysis Server.