Flex is for data, Flash is for graphics. (I haven't tried Flex.) You're going to need a way to create the graphical assets for your application, and as far as

Flex is for data, Flash is for graphics. (I haven't tried Flex.) You're going to need a way to create the graphical assets for your application, and as far as I know that means Flash - especially if you want vector graphics. The Flex SDK is free, but you have to pay for the GUI builder. I tried going "completely" open-source last year for a project with the FAMES (osflash.org) stack. It was a pain, but it had its benefits (mainly an actual IDE). It was still just for ActionScript, too - need flash for any kind of remotely complex graphics.