Well .. both are J2EE servlet container, and generate swf files on the fly .
Both are good solutions and to make them great, we have to get servers affordable.
Both are web applications deployed in a Servlet Container and they merely handles the compilation of the presentation layer and GUI event handling.
OpenLaszlo, is open source and it's absolutely free. Laszlo Systems, Inc, charges money for Consulting , custom development and support but Flex has a free Non-Commercial License