Tango is one of the common mandarins sold as "cuties" or "halo" and grocery stores across the country. It is very slightly pumpkin shaped and less round than the spherical "clementine" type that is also sold under this name. As far as mandarins go, I don't think Gold Nugget is all that similar to Tango at all. I've had some good Gold Nuggets from grocery stores, and some good ones from my tree, but overall they tend to be too puffy and over-sweet/not-sour-enough in my experience.
Tango is usually totally seedless, thin skinned, easy peel, sweet with a bit of sour and tasty. It is a great variety! But shasta gold is simply better in my opinion

Shasta gold has more cell toughness (flesh firmness), which I like. The rind is a bit thicker but easier to peel in my limited experience (I only got one Shasta crop so far and it was small). Mix of sour and sweet. I don't like pure sweet.
My Gold Nugget has always been a weak one. I think it is just my tree and not the variety in general so I don't want to comment as some kind of authority