You are really asking two different questions: SWT vs Swing, and Eclipse GUI Editor vs Netbeans GUI Editor (Matisse).
First, the difference between SWT and Swing is that they are two fundamentally different GUI libraries. This akin to asking the difference between Tk and Win32, or Java Swing vs .NET Forms (not to say that SWT is .NET). There are a lot of discussions out there discussing SWT vs Swing--I don't know enough about SWT to summarize the differences.
First, let me say my bias is in favor of Netbeans and I have spent 10 years learning the IDE from its days as Forte.
As far as the GUI editor, Eclipse and Netbeans have functionally similar products, but implement the code in very different ways.
My observation is that Matisse behaves, functions, and produces code that's reminiscent of Visual Studio .NET code. There are clear initialziation sections and custom behaviors for certain objects (such as the JTable). You can "Customize" an object and add your own arbitrary code via the GUI editor very easily for everything from initialization to setting individual properties. For event handling, it defaults to replicating "delegates" in .NET by using anonymous inner classes and invoking a stand-alone method. The GUI editor itself provides detailed access to the form object model and has a rich set of customizations. You also have the freedom to drop non-GUI beans into the form for use by GUI components, such as models (tablemodel, listmodel, etc), JPA-related objects, Workers, etc. What used to take a week to produce with hand-coded SWING takes a day with Matisse (though you have to really learn Matisse to do this). If you've been hand-coding swing for many years, then relearning to use a GUI editor effectively is going to be a long, hard lession.
The code is highly maintainable from within Matisse; it is NOT intended to be edited outside of Matisse, but the code is suitable for editing if you needed to (many folks I know use Netbeans GUI and then copy the source into Eclipse).
The Eclipse GUI editor is a very different creature. The GUI editor(s) are roughly the same in terms of overall capability, but I have found them to be less polished. The layout capabilities are about equal, though errors are a bit less forgiving at times. Some customizations required me to go to the source file and edit the file directly, rather than getting access to code customizations through the GUI. The code produced is very different than Matisse. GUI Components are added and initialized through "getters" and is scattered throughout the file; this good because each component is isolated/grouped into a single function, but troublesome when you need to diagnose bad interactions between component initialization. The same goes with the event handlers--very different than matisse.
Eclipse also did not provide any protections from me editing/breaking/tampering with the produced GUI file where as Netbeans was almost obnoxious with its protections. As far as maintainability, the Eclipse code is probably a little closer to the way a human would produce Java code... personally, I find the code it produces harder to maintain, but I've been looking at Matisse generated code since the first beta so my opinion is hardly objective on this issue.
Netbeans also has the capability of using the same editor for building Swing framework applications, Netbeans RCP, etc... I am unsure if Eclipse does the same.
