For me there is no comparison. The sigma is much harder to work with as it is so much bigger.
Mac 187 left, sigma 187 right.
Top: (dirty!) sigma 187,
Bottom: mac 187
To me they apply in a completely different way, and I feel the mac 187 is so much easier to work with.
I still think that for the price sigma brushes are great quality, but in terms of performance, I would have to say I prefer the mac brushes.