IntelliJ and NetBeans are both really good IDEs. I'm a NetBeans fan and I prefer having several projects open at a time. With intelliJ you can have only one project open.
And also as a maven guy I prefer NetBeans. E.g. download sources + javadocs is done with one click for dependencies of your project.
You can have multiple projects open in IntelliJ. When you open a second project, it prompts to open it in a "New Window" or "This Window". The confirm dialog has a "remember" option, which you can reset under Prefences -> General -> "Confirm window to open project in".
IntelliJ can also download Maven sources + javadocs automatically via Preferences -> Maven -> Importing -> "Automatically download". Otherwise, you can download manually by selecting the Maven Projects tab and clicking the download icon. Opening a Maven project with IntelliJ is really easy: just open the pom.xml and it will automatically create the IDE project.
And also as a maven guy I prefer NetBeans. E.g. download sources + javadocs is done with one click for dependencies of your project.