Duo fiber brushes have been mostly a miss for me, but that 159 looks interesting. I like the shape... saving that one for later in my mac profile.
I LOVE the 159 - it's become one of my favorite highlight brushes.
In the past, I liked the 131 the most and still do when I want a nice wide sweep. It's also my hands-down favorite for bronzer. I'd also use it for deeper MSFs that I wanted sort of a bronzer/blush/highlight all-in-one effect with. I wish it wasn't LE, because it's a very versatile brush.
Then when the 128 split fiber came out, I started forsaking the 131 a bit, especially with extra-shimmery highlights like the EDSFs and CCBs, because you could control the placement really carefully.
Then, when the 159 came out, I just about jumped ship from either the 131 or 128 because this brush is just beautiful. The white bristles are just sparse enough that they trap and remove excess glitter out of really sparkly MSFs, but the belly of the brush is so thick, rounded and dense that it grabs plenty of product in one charge, unlike other skunk brushes. The nice dense belly feels so soft and luxurious and the size is also perfect for careful placement, rather than big swoops, but bigger than the 128, so it works with less metallic products, as well as with shinier ones. It's big enough to sweep with but not so big you can't pat with it too. Because it's nice and dense, you can buff and blend with it too, not just apply, which is something the 128 is a little small and skinny for.
At this point, the 159 pretty much knocks the 128 out of the park, except that the 128 is great for creams as well as power, plus the 128 is best for the most concentrated placement. I may just start switching my 128 more over to concealer and moisturizer, but I do swear by it for stuff like RiRi Diamonds and the most metallic EDSFs.
NOTE: Like Erine was saying, the 188 is super versatile too, and great if you like to swirl-buff your highlight into your foundation. When I didn't have a lot of brushes (or highlighters), I swore by the 188. It's still my primary blush brush and I use it just about every day.