Re: Brass vs Tungsten or both
As an option, you can tie your nymphs w/o beads and then slide the bead on the leader prior to tying on the nymph. The bead will slide down and essentially seat itself ahead of the nymph- the action isn't the same as when the bead is an integral part of the fly, but it's close. In areas where the water is extremely clear of the fish are very picky it might make a difference, but if the fish are sucking in beaded flies, you're probably pretty safe.
The big difference (as others have said) is how quick the flies sink... tungsten beaded flies drop REAL fast!