Flex vs AS projects

I just read about using Flex projects (MXML) vs AS projects in Flex/Flash builder, and Flex project’s final swf is a lot bigger (which makes sense), but this is worth it sometimes for the ease of menus and such. However I read that compiling with Flex gives a slower performance, even if you don’t take advantage of any of the components. Does any one have any experience with this? Which to choose?