<p>I want to play around a bit with FLASH for app development. I'm looking for a good IDE for that.  Someone suggested Flash Develop.</p>

Major reason to use FlexBuilder: has a real debugger you can set breakpoints and single step and watch/edit variables.
Unless FlashDevelop has .. developed .. don't think it got any of that yet.
BTW - if you are a student/teacher FlexBuilder can be had for
free