I tend to think the right answer depends on the culture of the target platform. On OS X, Interface Builder is such an integral part of the tool chain that it's difficult to avoid it.
In Java (awt or swing), the reverse is really true. There is no toolchain support for this.
Really, the way you can tell, is the way the tools produce their outputs. Interface Builder produces .nib format files which are specific to the way Cocoa puts controls on the screen. It understands what is essentially an interface markup format. Java has no such similar concept. Everything is a compiled class, and therefor its much more difficult to get convenient results.
GTK+, when combined with glade, seems to strike a reasonable balance between the two.
