Not saying the manfrotto ones aren't steady, but the arca ones have more to offer, being a 'standard' of sorts, various makers and plates are available eg - L plates, replacement foot plates.
As opposed to clamp designs, which can be rather limited, but still an option for video makers, arca has more flexibility IMHO.
Not to mention, any arca ballheads can easily take in the plate you have, without changing. Clamp styles are pretty hard to find an exact plate or have others using the same plates which can interchange with the clamp.
Just my 0.02c.