Re: Sulfur nymph vs March Brown
It may be just a "confidence in certain fly types" thing, but for me the simple Pheasant-tail nymph in sizes 14-16 does a great job of imitating the sulphur (Ephemerella) nymphs.
When fishing Pocono streams with good March Brown populations, a standard Hare's Ear nymph (no beadhead) with exaggerated legs (brown partridge hackle) in sizes 10-12 worked much better for me this time of year.
"We fish for pleasure; I for mine, you for yours." -James Leisenring