Go for both
As you have said you are very good at Flash ActionScripting... You don't know you are already good at Flex too. What you need is a basic introduction to be familiar with Flex, thats it. You'll start building RIAs in the next moment.

AIR is the future of desktop applications. I have seen some of the coolest apps built on AIR technology. Its definitely gonna in demand in very near future so its good to learn AIR now.