nera_pooh: what malavikat said....but also, if you're using a thick foundation like the Studio Sculpt, then you might like the 109 better. I find the 187 to be better with thinner or more liquid-y foundations since it's harder for the white fibers to buff in thick consistencies.
Review on NEW SS109:
Ok so I've been using it for a few days now. The new ss109 is the same length as the mac 109. I washed it as soon as i got it to see if it would fan out as much or more than the mac 109. The one i got fanned out more, and is more fluffy. Hence, it's not as dense as the mac 109. However, it's not to say the SS109 is not dense. It is, but it should probably be more dense since the 109 is suppose to be a really dense brush. Even when i applied it to my face, it didn't feel as stiff...it felt more soft?
So for those who like to buff their foundation in with a 109, you might prefer the mac one more, as denser brushes are better for buffing in foundations. If you just stipple your foundation on, then i think either one will work. I didn't notice a difference in terms of using it for stippling foundation on; only for buffing.
I think for the price difference, the SS109 is workable. It's definitely not a terrible brush. If you have a mac 109 already, you may want to stick with your mac one, but if you're looking for a cheap alternative, then the SS109 is a good choice. Again, the sigma one is $14 US, while the mac one is $38.50 CDN - sorry! I only have the Canadian price for the mac 109. With a 20 something dollar difference, i say why not try the sigma 109 if you cannot afford the mac one.
OH, and about the flat top, it's not as dead flat as the picture shows. when it's all puffed out, it's just as flat as the mac one i find - well at least the one i got is.
HTH!