Of course, there are a number of programming style considerations where I consider the OpenLaszlo conventions to be simpler and superior to Flex, but these are just a matter of opinion. But the undisputed advantage that OpenLaszlo has over Flex is that OpenLaszlo supports multiple platforms: Flash, DHTML/Ajax, J2ME and other upcoming web platforms.
This allows developers to maintain a single set of source code that can run on multiple platforms. While Flash has extensive market penetration, there are still market segments where it isn't supported, for instance the iPhone. In addition, many people don't want to support the Flash platform because it is a closed and proprietary platform. Your OpenLaszlo application won't be locked out in this manner.