Which would be more comprehensive for game development?
flash is aimed at designers.
flex is aimed at programmers
so it depends on your function in the game team.
Since HTML5 is the future, you might also want to check out Processing.js, if you don't mind considering alternatives to Flash.
Processing.js uses Javascript to draw shapes and manipulate images on the HTML5 Canvas element. The code is light-weight, simple to learn and makes an ideal tool for visualizing data, creating user-interfaces and developing web-based games.
HTML5 aims to reduce the need for proprietary plug-in-based technologies such as Adobe Flash, Microsoft Silverlight, and Sun JavaFX. (Wikipedia: HTML5)
In this case, you would have a vast choice of Editors or IDEs.
Use Flash for games. Games involve more animations (suits flash) than data driven user interface controls (suits flex).
The editor in Flash is terrible. Flex Builder is based off of Eclipse, so if you're used to / or like Eclipse, you'll probably like Flex Builder. But I'd say neither, and give http://www.flashdevelop.org a try.
It depends.
- Are you working by yourself or in a team ?
- Are you more a designer or a developer ?
- What's your level of comfort with actionscript ?
If you're just starting with actionscript, you're somewhat comfortable with the Flash IDE and you want to make fun indy games for people to bash buttons and destroy mice, go with Flash. The look will weigh more at this stage and interaction will be simpler. Never the less you can create some addictive games with very simple interaction.
If you're more of a developer, I would suggest using Flash to generate your assets (characters, animations, etc.), or have your designer do that for you, but use a proper IDE to write your code. As Flemish Bee Cycle mentioned, you can try Flex Builder or if you're on a PC, Flash Develop is an awesome IDE( free, lightweight, fast ) You could try to write you're own mini game engine, or use an existing one, like the PushButtonEngine.
As far as I can see SWF is the dominant output for online games:
HTH
Do the code in Flash Builder, as it is a better tool for coding; and the graphic resources in Flash.