in my experience, off fifth has way better selection, ie more styles and more in those styles. whenever i go to nr they tend to have only a couple bags at most (if any) and there's only one of each. that said, nr is, i believe 50-60% off retail while off fifth is 33%ish, so if you get lucky and find what you want at an nr, you're set!
off fifth emails out coupons occasionally and all the lovely guys and gals of this forum usually post them up, either in here or in the deals&steals thread. sometimes its a print-out coupon and sometimes its a in-store kind of deal. if it's a coupon, yeah you just bring it in to the store, or if you're doing a phone order, just mention you have this coupon and they'll have you either fax it in, read off the code or they'll just take your word for it, depending on the sa.
hope that helps