These brushes definitely function the same way. I want to say that since the Sigma one is less dense and more tapered it would be better for actual contouring, which is what it's intended for, and using cream products such as cream blush. And the MAC one since it is more domed and rounded is better at liquid foundation, it can swirl well and tap the product. I think Sigma did a good job here overall of trying to duplicate this brush though.
Have do you own either brush and how do you think they compare?