I am torn between Wicket and Vaadin. I am starting a micro-isv and need to make a choice of web framework. I have narrowed down my choices to Wicket and Vaadin. I have used both
I have a limited amount of experience with each but I prefer Vaadin. It allowed a richer experience with the web application I was developing. The main benefit that sold us though was how easy it was to write unit tests around our UI classes, ensuring the components functioned correctly when interacted with in the expected ways. This is also possible with Wicket however it was more difficult in my experience.
I will also mention that either framework will require some styling. Wicket starts off as plain old HTML and Vaadin starts off with a MacOSX-like theme by default but almost any web-app you write will require at least SOME customization. With this in mind, customizing the CSS of a Wicket app is SIGNIFICANTLY easier than Vaadin for the simple reason that you control the markup. Vaadin hides the markup from you and generates elements with weird IDs and structures so it is harder to customize the look. Just remember this when making your decision.