Somewhere on vimeo there's a comparison video and the Sigma seemed to come out ahead. I have the Sigma and love it. My only issue is the slight barrel extension (about 1/2 inch), but it's easy to remedy if you use a mattebox with nun's knickers.
crop factor doesn't change the main rule about wide and telephoto lenses.. it will never give you the same picture as 35mm lens on FF.. unfortunately. so... if you are going to use bmcc for the film production, 8-16 waste of the money.