please consider that the size (in KB) of the swf file will be significantly larger if you build it in Flex, rather than Flash.
Thanks. Could you explain why this would be?
Flex is a framework. Although the output of both Flash and Flex is a .swf file, if you build it using the Flex framework the output file embeds components (e.g. button, scrollbar, text field, canvas), which probably you will not need if your goal is to create a banner.
E.g. the smallest app built in Flex is ~ 170000 bytes.