I have been using Flowmotion on my GH2 since Jan, and its been rock solid. I tried a few other hacks now and then but they were all too buggy, and IMHO I didn't really see any improvements over Flowmotion. Moon was no good for me, as it saved corrupted metadata in the files and FCPX refused to load them. So if you want a really stable hack I would recommend FM.
My humble opinion is that they can't be compared. Apples to Oranges. Moon is very, very good as are most of Nick's settings. They are geared for more specific shooting situations though.
Flow Motion is also very good but more forgiving (Again IMO) and better suited for general applications.
720-60 which is the Achilles Heel of the GH2, seems better with Flow Motion thus better for outdoor sports.
I think a better comparison would be Sanity Vs Flow Motion.
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