I have all of the brushes you mentioned...
If you just want to dust a little powder to set your foundation the 150 and 134 do the same thing, all though the 134 is bit more dense so you might get a "slightly" heavier coverage. I dont know.. In all honesty i can use both of them for powder and blush application and i see absolutely no difference lol. But the 134 is soooooooooo amazingly soft
Now.. my 136... i spent $62 on a brush i never use. I hear you can apply blush with it if you want a sheer application, apply loose powder, and even use it for a bit of conturing.
The 138... I like the baby version better which is the 165. This brush is super soft, its very tapered, you can apply blush and contour your cheeks with it and use it for highlighting. But i think its skippable, i never ever reach for it..
Why do i have all these brushes????
Im just a huge brush and pigment addict i think.