After asking the same question to 10 different people, I found that depending on your flying skills and how far away you want to be from the target, you should choose different lenses.
I use .3 (Raynox 3030) for freefly films (1-5meters away) and .5 for other purposes. You can watch 0.3 film at: http://www.vimeo.com/9274549
/Karl, Sweden (beginner at this) CX 105E in a Tonfly CC1