Both brushes have comparable handle length, and bristle colour. You can see below that they are of a similar size, so if you liked the MAC 187's size, you'd probably like the Sigma Flat Top Kabuki as well. The quality of both brushes is also very good - I have to admit that I wasn't expecting much from a Sigma brush - after all, nothing lives up to its hype, right? - but I am forced to admit that the Sigma brush is actually of much better quality than I expected.
Top: Sigma Flat Top Kabuki, Bottom: MAC 187
However, despite their similarities, they perform totally different functions. After all, the MAC 187 is a duo fiber/skunk/stippling brush (whichever terminology you prefer), while the Sigma Flat Top Kabuki, is well, a kabuki/buffing brush. In short, this means that the MAC 187 would be bettr for applying product in a sheer, sparing manner (e.g. for heavily pigmented blushes or foundations), while the Sigma Kabuki would be best for buffing, or blending (e.g. blending after all product is applied, or buffing in mineral makeup). But let's explore the brushes a little more.
The MAC 187 has bristles that are a lot less dense than the Sigma Kabuki. So for more pigmented products, the 187 might be a better choice, since less product will be picked up and deposited. However, for sheerer products, the Sigma Kabuki would be better, as it can really pack on product.
Also, the MAC 187 has natural bristles, as opposed to the Sigma Kabuki which has synthetic bristles. While there are no hard and fast rules for what brush goes with what product, it has been my general experience that natural bristles do better with powder products, while synthetic bristles work better with cream, liquid, or gel products. With natural bristles, they deposit powder products evenly, but if used with liquid products, the bristles tend to absorb some of the moisture from the product, making application uneven. On the other hand, synthetic bristles tend to work well with liquids and creams because they don't absorb any moisture, however I do find that for some reason they don't deposit as much powder as natural bristles do. For some reason the powder just seems to get "trapped" in synthetic bristles, but not in natural hair ones.
So, although they might look similar, they actually aren't when it comes to function. The MAC 187 is better for powder products, and for products that are more pigmented, while the Sigma kabuki works well with cream, liquid and gell products, and also for products that are more sheer. The 187 is a stippling brush, while the kabuki is a buffing brush. They actually complemnt each other very well in that regard. Well worth owning both, in my opinion.
(The Sigma brush was sent to me for review. Review is my own honest and complete opinion. I am not affiliated with/compensated by the company for this review.)