1) JQuery is built upon a simple principle of "progressive improvement".
- You start from a basic design and keep enhancing it. This approach is very productive as you start with a basic page design and then add functionality by using the selectors and defining the behaviors
2) JQuery Code is simpler and easier to understand. We shrank some of our 10K line script files to just a 1K or so. Significant amount of modularity is achieved using the plugins.
4) Service invocations: One of the biggest advantages I saw with the GWT was that making REST based calls to the Server became so easy. GWT-RPC makes those rally easy and straight forward, the only rider being the GWT needs a server piece to do it with GWT-RPC (Your Project is not all Script I mean). At least in some cases we had a bad taste with this as the Existing Server application was written on JDK 1.4, and to get RPC running we had to make it run on JDK 1.5. Do not think this is a big problem, it is high time we upgraded. JQuery provides a robust AJAX call framework, gives very good options of hooking up to most of the callbacks. One thing I am seeing is that as I use completely Script, the server side is decoupled of how I provide REST based services. This is not to say GWT is not capable of doing this, it can very well do the same using the HTTP Request Infrastructure. But rather than writing GWT RPC Services, we are now preferring to write our REST Services using JAX-RS or Grails, which decouples the Server side from what Scripting framework is used on the Client.
5) JQuery has very good templating support now. Most templating engines are available as plugins and can be used out of the Box, with JSON data fed in to them. Makes life really simpler for complex objects presented on Repeating Divs, Grids etc.
6) I found it kind of difficult to debug the generated code from GWT (Even in the detailed version). The Java debugger on Eclipse Helps, but my firebug or my server logs did not give me much of a clear picture when an RPC call failed. May be I am not smart enough for that. JQuery code was mine, debugging turns out to be simpler.
7) Plugins for JQuery has increased many fold int eh last year, we could virtually find ready made functionality for whet ever crazy stuff we wanted to do, Grids, Grids within Grids, Slideshows, Graphs (Bar, Pie, Line -- what not ????)
8) Editors : Google launched the GWT Designer recently which I believe should make a world of difference to the way the GWT pages are coded. This should make building the UI and even positioning of elements much more simpler with GWT. JQuery Visual elements being nothing more than HTML Markup, will always have advantage of using of any of the HTML WYSIWYG Editors out there.
I definitely think both GWT and JQuery are really powerful tools. GWT might be the best choice for people coming from a Java background and want to still program with the Strongly Typed language with good compile time checking. But this requires a lot of unlearning of Java as well as to what cannot be used on the Client side code
JQuery on the other hand, expects you to know HTML, DOM Structure and Event handling but is simpler as a framework. I strongly believe that unless you have a huge pool of Java (Only) Developers who need to be used for Web GUI development, all other cases it might be worthwhile to consider using JQuery instead.