Here's my basic checklist for deciding which approach to take:
jQuery+HTML+CSS wins if...
- You care about SEO
- Your application is primarily oriented toward HTML design & content
- You want a very high degree of support for browser-specific extensions, and you want to spend the time testing and supporting those extensions yourself
- You mainly have a background in web development and want to add "a little bit of interactivity or Ajax"
GWT or ZK wins if...
- Your application is based on a desktop metaphor
- The bulk of your application is based on frequent updates between the client and server in lots of places in the UI
- You are ok with the mobile support provided by the respective framework
- You mainly have a background in server-side development and really hate the idea of dealing with all the different browsers (you just want a framework that abstracts that away).
- You don't plan on building a lot of custom components - the ones the frameworks come with look pretty good to you.
Other component-based frameworks include: