This week's giveaway is in the EJB and other Java EE Technologies forum. We're giving away four copies of EJB 3 in Action and have Debu Panda, Reza Rahman, Ryan Cuprak, and Michael Remijan on-line! See this thread for details.
Without having used either of the two extensively, I'd try to prototype the difficult parts of your UI in both technologies (or try to find samples where people have got features working). You might quickly find that one or the other falls short in some area.