I'm with Mark Libertone (Softhackle). I like the 3399 for wets, and prefer it to the 3906. I've heard some say there is no real difference but to me there is. Without getting into specs and eye lengths, for me the real difference is at the eye. The 3906 seemingly has a tiny "ramp" where the shank leads down to the eye, and the 3399 doesn't, and I like a wet fly hook that doesn't. The 3906B I always thought of as a nymph hook, and it's longer. The slight ramp down to the eye is quite problematical for me, and my thread always seems to go down there and get caught in the eye. Maybe it's simply the shape of the eye, I don't know, but there is a decided difference, at least for me.
Eric