Both of these golds are highly transparent and dispersive, and both are made from nickel. Nickel Azo Yellow is more of a yellow – warm and ochreish in masstone and cool and lemony in dilute – while Rich Green Gold is more, well, green! It looks to me like pickles. Both make more muted, naturalistic greens with Phthalo Green than a bright yellow would.
For my money, they have roughly the same role, so which of these should I pick for my palette?