I have bought a DVD over a BD even when I can easily afford the BD and have the equipment, but in those cases whether the price difference was £3 or £13 wouldn't have changed my mind; I was always picking the DVD for some specific reason beyond that. FMA may have been affected by people calculating the total cost of ownership over the series, true, but I'm not convinced many buyers think that far ahead, either.
Perhaps it's just a slight difference in the demographics both appeal to (FMA is undoubtedly more popular online, but how much of that translates into people who preorder BDs is unclear), and perhaps with FMA there was some confusion about whether the series was new or a repackaging, because remaking a successful non-vintage TV anime doesn't happen every day. MVM are also more specialist so a BD outselling a DVD on preorders (a specialist method of purchasing something in the first place) when it's being promoted most through smaller, more niche stores/publications, might make sense. Maybe. Good luck to them anyway!
Maybe one of the reps can be persuaded into revealing a general DVD:BD performance comparison for Samurai Girls if someone asks nicely.
R