The trick for all successful breeders is to supply their market needs, which in turn, should supply flies that the fish are attracted to. All of the qualities in the cape or saddle- feather, barbs, stems, etc. mean nothing until they are palmered around the hook on a fly pattern. All of the qualities of good dry fly hackle need to be there upon scrutiny. So thereafter it boils down to color, and that is open to a lot of subjectivity. I know of two traditional patterns that used brown and grizzly feathers together, and that is the Adams and the humpy. There are probably more, and then the variations of other patterns that are 'enhanced' by cree. But if you tie an Adams with a 50-50 balance of brown and grizzly, you get a 'look' as you hold it at arms length. Then compare that to an indentical pattern using cree instead of the two colors, and you should get the same look. One of the 'stipulations' of a true cree is that all of the colors are represented on each barb- black, brown, ginger, cream. While the fly is hi-floating, I don't think cree coloration matters to much. But as the fly starts breaking the surface film and is drifting lower in the water, more of the color is apparent to the fish. Now whether brown/grizzly or cree is more effective is open to debate and makes necessary many and repeated trips to the rivers and lakes to test all suppositions and presumptions. And then those outcomes need to proved again to make sure.