• Flex/WPF both use XML for vector layout.

  • XAML (WPF) is a published opened standard, Flex's (sorry, have forgotten the name) is a one-off. In fact, there's already a heap of support with XAML in the convertor department, like XAM3D (ala SWF3D).

  • Working with WPF means you can code in .NET and work with a compiled object oriented language. Not sure if Flex uses Actionscript 3.0 or not, but pre-3.0 reminds me of VB in that it permits you to make stupid mistakes. In addition, standard stuff like regular expressions and xpath are unavailable pre-3.0 as well (yes I know about XPathAPI - but its a really limited implementation, and it doesn't work most of the time).

  • I like the animation systems of both.

  • If working with WPF you can create your webservices as another project in the same solution. If you work with Flex, you need to work with 2 different IDE's. I know that's not much of an argument, but hey.