Google Web Toolkit (GWT) provides a development tool chain for building and optimizing complex web applications, and is used in products like Google Wave and AdWords. Closure is a set of individual JavaScript tools that are also designed to help developers build complex web applications, and is used by Gmail, Google Maps, and Google Docs.
Closure is geared toward developers who want to work with JavaScript and have a strong understanding of the language, while GWT allows developers to work primarily in Java (although they can work in JavaScript, too) without worrying about the underlying JavaScript.
Wow. that was fast! ( I was about to curse myself for asking without
having properly looking over the documentation, but I happened to have
the page you linked to open in another tab from 3 minutes before I
posted, and your answer was not there at the time.)
More questions..
So the standard libraries from the two projects are not related ?
Is there overlap/sharing of the optimization / compiler code ?
How does the compiler compare to something like YUI compressor ?
( http://developer.yahoo.com/yui/compressor/ )
I think the unfortunate answer is "it depends" :)
Language familiarity isn't the sole reason to choose one or the other.
Each is a fundamentally different tool, and your choice should depend
on your requirements, developers' language familiarity, what you plan
to build, and how you want to build it.
There are projects that use each within Google, and each could give
you a good reason why.
Nathan