If you want to see a side by side comparison of three flies tied with three different yarns including the two above click on this post in the Fly Fishing Forums. If these photos are correct there is not much difference between the three once the flies are wet. A friend of mine recently posted about this fly on his blog Fly and Fin and recently I watched an Oliver Edwards DVD were he tied and fished the pattern. So I decided to tie up a few myself. I tried to follow Frank Sawyer's instructions to the letter including using two layers of wire and three layes of yarn, tying the fly off at the rear instead of the front.
Bottom line...none of this makes a difference. All this nonsense about materials doesn't matter. Tie some grayish, pinkish tan yarn with brown high lights on to a hook and go fishing! It's a simple pattern that works very well.
This fat rainbow could not tell the difference in the yarn colors either!