I think you will find more support for using the Java applet in the user interface instead of Flex. By support, I mean libraries, forums, and tutorials. I've been looking at Flex for quite some time, and the search results that I find are less focused on developers and more on non-developers trying to do beautiful things. I believe that there is nothing wrong with Java applets as an interface, and I think more people will come when Java reduces VM startup time (current priority for the next version).
If Java applets are in dispute, then there is the Google Web Toolkit (GWT). It provides full development, testing, and debugging in the Java development environment, but is then automatically deployed to HTML / JavaScript. Absolutely fantastic. Their third-party components have indeed come a long way, and their user support makes the choice easy.
Fewer languages ββ= fewer mind swaps.
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