I only worked with Trinidad and Richfaces. I like both, but I would give an advantage to Trinidad. Perhaps because I worked with him more, and I feel more comfortable with him. Trinidad's online documentation is also pretty good.
Both Trinidad and RF have a component library. You can check these links to find out which components you find useful for your application. I used the Trinidad tree in one recent application that I developed, and it was easy to use and populate: Trinidad Components - RichFaces Components (Login required)
Regarding skinning, I think the default Trinidad skin is by default simpler than RichFaces, but I think that both have ways to make them look as beautiful as your CSS skills allow.
I think RichFaces has a more mature AJAX integration, and their AJAX components and events are more convenient for me than Trinidad ones.
Averroes
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