I personally prefer ActionScript over Flex due to many benefits ActionScript has for me, below are a few:
Achieve greater performance
ActionScript 3 was written from the ground up with performance in mind. Depending on the content, you can see a significant increase in performance. This means that your existing content may run smoother, and your new content can do more, while using the same amount of CPU resources.
Leverage new Flash Player APIs
Adobe Flash Player 9 and 10 includes a ton of new features which can only be used through ActionScript 3. These include much easier XML APIs via E4X, more advanced display list manipulation, or doing advanced image manipulation with Pixel Bender filters. As a general rule, new ActionScript-based features added in the future will only be available via ActionScript 3.
Leverage community libraries and APIs
Almost all of the major new libraries released by the community are built with ActionScript 3, and include everything from the Papervision3D full 3D engine library to Grant Skinner’s Gtween animation library.
Troubleshoot code more easily
The ActionScript 3 compiler provides options for much stricter error checking, which means it is more likely you are going to find bugs and errors before you even begin to run your content. When you do find errors, you can take advantage of some of the new and more advanced debugging features to track them down (and of course, you can still use trace()).
Develop content for multiple platforms
ActionScript 3 is the standard language used across the Adobe Flash Platform. Moving forward, it is the language that Adobe will focus on supporting in existing and new players (like Adobe AIR), servers, and products. Flash Player 10 supports ActionScript 3, and its use is required for developing for Adobe Flex and Adobe AIR content. In addition, Adobe is working on updating its mobile runtimes to ActionScript 3. In the future, you can expect that new Flash Platform products, runtimes, and services from Adobe will use ActionScript 3.