FlashDevelop.org - Welcome. Flash vs. Flash builder?? I'd recommedn, in order to find out what both programs are like, searching online (or YouTube) for videos about them. look for things like "intro to flash" or "intro to flash builder".
It seems basic, I know, but for being a new comer to both programs, I think seeing some of the hundreds of videos that are out there could have some value. I'm a Flash developer, and have very little experience with Flash Builder, so I'd be biased with saying "go with Flash". Actionscript 3 - Conceptual differences between Flash CS5 and Flash builder 4.
Understanding new Flash Catalyst CS5.5 and Flash Builder 4.5 workflows. Adobe Flash Catalyst CS5.5 and Adobe Flash Builder 4.5 have a host of new features that enable workflows for building engaging and expressive Flex applications. The workflow you use will depend on the kind of team you have and the size of the application you are building. Your first option is to use Flash Catalyst as a wireframing tool. As a wireframing tool, you can rapidly create application layouts, choreography and interactions. If your goal is simply a wireframe then you might stay exclusively in Flash Catalyst CS5.5.