Flex is a perimeter game that relies more on BH than PAS. Motion requires more Pas than BH. You can usually get away with lower levels of PER and LP with Motion and as Bbunch said, a fairly even Dist is most affective, whereas with Flex, you can set wider ranges of Dist and still be affective and you want a lot more PER. Motion also requires the highest level of IQ, and IQ is not as important for Flex. I always felt you needed A IQs for Motion, but could get away with B IQs for Flex, even C for younger players.
All of this is based on Hoops 100, FAQ, and Critical News threads, a couple of which are no longer there. I spent a lot of time analyzing all the offenses and defenses 5-6 years ago. The problem is that there have been many upgrades since any of this data has been updated, so I don't know if any of this is accurate any more.