First of all comparing GWT with jQuery doesn't make much sense. While jQuery was build to make cross browser working with the DOM much easier, GWT is build to create large web applications.
So if you a bunch of static sides with some independent widgets, like calendar, slider and so on, jQuery is enough. If you want to build single page app, maybe with a large team, GWT is the better way. GWT has a lot well design architecture under the hood, especially the build in MVP pattern, UI-Binder templating system, i18n support etc.
So as JavaScript developer working more than year on a large GWT app, I would recommend never ever build a single page app only with jquery cause it wasn't build for. If you wanna use JavaScript take a look at backbone, spine, knockout or dojo.
Btw, as good the GWT architecture is build, you will have a lot of JAVA overhead. So if you project is growing, the time to compile your css and i18n properties will come annoying.
