Flex builder for me. It is the easier than Flash.
Flex Builder is best for AIR apps. Flash in general is not good for building apps, but is better for animations.
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I faced a similar choice recently: Flash or Flex for developing CS4 panels.
John Nack advised me to use Flex Builder. He was right. For a desktop AIR application or something like CS4 panels, the Flex Builder IDE is more powerful and helpful. If you're a developer familiar with other programming tool IDEs, like Microsoft Visual Studio, various Java IDEs, etc. then you will find the Flx Builder IDE to be more intuitive.
Flash is a good choice for developing animated SWF panels that are driving things like Flash movies. If you use Flash, you'll have to deal with the Timeline. With Flex Builder, you don't.
They both provide access to Action Script. But they expose functionality differently. Also, Flex Builder provides some data visualization tools, etc. that are not part of Flash.
thank you so much!!
I will use Flex builder.
Glad I could help. Flex Builder is an excellent excellent development environment.