In the last half year I spend my time working on a big project in flex/flash environment. There is always a dilemma - How to create a GUI object: with MovieClip or flex component:
If you are choosing flex, then you got a huge library of system components that you can use as strong staring point. The bad thing about it, that it is not so easy to work with sparks, every small change that you are doing force you to create 5 new objects that will not always look at the end as you design.
if you are choosing flash, then you can do what ever you like without any limitation, the only problem is you not always know what is the best way to do it, working with system components in many cases will improve the performance as it been tested a lot before the release.
And now comes the most tricky part, what would be the best way to implement animation.
Using flash classes is not an option because the mx and the spark libraries got much better deal to offer, but the true question is if you are going to create the animation in a script or you will use flash time line instead.
The best thing in flash is the time line! It is much easier to create animations with and you can edit and test your motion in real time.
How ever writing a script will provide you the same tools as in flash and some times even more, (you never know what your Tween will give you),
the only problem with scripts is when you want to edit them, to test what you did you need to compile and run the application witch can be a nightmare some times, especially when you are doing it more then 100 times and it's still doesn't looks right(and as you know flash builder compiler like to take his time).
Off course that if you got same motion in 10 places you will not do it in flash, fixing one will force you to fix 9 more, but usually is not the case.
Tell me what do you think about it, how do you work?
I think you have a very good grasp of the issues and you, like the rest of us, have to give it a shot and hope we make the right choice, given the requirements of the app. I'm serious. There's no correct answer. If you want lightweight, you almost have to go with Flash (though you might check out Reflex) but with higher powered machines and a faster Internet, life can be easier with Flex and quicker development is important.