Duo-Fibre brushes are a mixture of two types of fibre, usually goat and synthetic, though two types of synthetic can also be mixed, like in the case of the Real Techniques Stippling Brush. This mix of fibres allows for the bounce and resiliency of natural fibres with the softness of synthetic. This mixture makes them good for stippling, as the bounce the goat gives heft to what would otherwise be a synthetic brush (which sometimes has the tendency to splay and flop). Therefore, they are popular for foundations but are also good for cream and powder blushes, mineral products, bronzer, and anywhere you'd like to make a pigmented product go on more softly. Because of the similarity of these brushes, I'm mostly going to talk about size differences (flashbacks to the very first brush post on flat-shader brushes....).
Large Duo-Fibre Brush
Here we have MAC's 187 brush. I apologize that it is stained pink, but it won't come clean no matter how many times I use cleansing oil on it. At least the staining indicates for what I use this brush most, which is blush. It's great for a large blush application of a very dense blush. I think this staining might be Illamasqua Panic. It's also a popular one for foundation, though I don't quite get the stippling of foundation. As I am theatre trained, for me, stippling is used to add natural veining effects, or to tone down shading, or add more red tones to old age makeup, places where the goal is to retain or create a texture, exactly the opposite of what you want in your foundation. Then again, though I've heard of people claiming to stipple their foundation with these things, I've only seen people use these to buff, which in my opinion is better done with a denser brush.
With powder or cream color cosmetics the stippling can create a translucent blush effect, so these brushes are great for that. They also work well for a very light dusting of bronzer or contour. They are often marketed with mineralized makeup, like MAC's Mineralized Skin Finishes, since those type of products can deposit too much product on denser brushes. The large duo-fibre brush creates a diffused application over a large area.
Medium Stippling Brush
The Real Techniques Stippling Brush is much denser than the 187, it also has a shorter distance between the black fibres and the white fibres, both of which are synthetic. This makes it work more like a flat top buffing brush than a stippling brush. This works very well for foundation, as well as cream blush application, but, unless you want a very dense application, I wouldn't use it for powder blush.
Small Stippling Brush
This brush, MAC 188, is a thinner version of the 187 above. It has a farther distance between the two types of hair than the RT brush, so it is a bit floppier as well as thinner. A lot of people also use this brush for liquid foundation. I prefer it for blush, and often apply with this and blend out with the 187 for a bit more control than the 187 offers. It's also great for highlighting cheekbones.
Short Stippling Brush
The MAC 130 is probably my favorite of all of these stippling brushes. It's wider than the 188, but very short and denser. It has fewer hairs I'd say than the RT, but since they are shorter, they are a bit stiffer nonetheless. I imagine it would be great for cream foundation, as MAC often markets it for, and I absolutely love it with cream and liquid/gel blushes. If I'm wearing a cream blush or a MUFE HD blush, I've likely applied it with this one.
Pros and Cons
Pros: These brushes are versatile and useful. The less dense, longer white bristled brushes are great for applying powder products sheerly, while the denser, shorter brushes are great for liquid and cream foundations and cream blushes.
Cons: I don't think the aforementioned less dense brushes are good for liquid foundations. The largest can be unwieldy to use for blush, unless you have a larger face or like blush over a large area--like I do.
The Brushes featured in this post are the MAC 187 Duo Fibre Brush, Real Techniques Stippling Brush, MAC 188 Small Duo Fibre Face Brush, MAC 130 Short Duo Fibre Brush. All were purchased by me and I am not compensated for featuring them.