I'm going with developing proven skills. Although, its a bit convoluted. Prime example, Ruby on Rails is (was) a hot future trend. But it's based on a proven programming language, Ruby.
In your case Dennis, the choice between those two could be a crossroads as a developer. Flex and RoR are two different frameworks for different purposes. The saying "The right tool for the right job" applies. You wouldn't try to hammer a nail with a chainsaw.