For me, being able to find developers for the project, when other developers leave. There are a lot more people that know Swing than SWT for desktop applications. I actually have never seen a desktop app written in GWT, I know there are some and I liked the idea of the "eclipse framework with SWT" for making your apps, but I tend to choose technology that I know I can easily hire people. So whiles it might look a bit nicer, I know that Swing's api is pretty easy and makes sense, and that there are some great Swing developers out there that makes great looking UIs. Including my friend Ahmed, speaking of which he is in Southern CA, if you need a Swing guy, you got to get Ahmed to help.
Anyway, that is my opinion, I think that there are lots of factors to consider when deciding any technology. But personally for all Desktop Apps right now, I would choose Swing. But based on your comments, I am predicting you like SWT better and will go down that path. ;)