Is there any benefit these rare planets give you over common planets? In other words, do you get the same out of an average baron planet with extremely abundant mining as you would get out of an average metallic planet with extremely abundant mining?

If they are in fact the same, why would you use your hyper terraforming artifact on a worthless common planet that hundreds of people might scan and steal from you when you can use it on a rare planet (even an Oceanic) that will rarely be scanned?

There is no difference between 2 planets of the same size and abundancy, regardless of whether it is rare or not. rarity is defined by the number of them in the galaxy, not by how hard it is to find. however, the rarer planets are more likely to have higher abundancies and sizes.
So an average sized, very abundant terra is the same as an average sized, very abundant barren; but barren planets are more likely to be average while terras are more likely to be rich.

Heh, I have 3 Terra planets scanned out right now, I only possess one of them (the other two are not colonized) because they're pretty meh, even being Terras. My best planets are all Metallic, Icy, and Volcanic. Especially love my Metallic planets...ultra rich massive ftw <3.

"Difference between Metallic, Exotic, Terra, Baron?"

Easy answer.. Baron is not a planet. It's a title of nobility so it sets apart from metallic, exotic and terra category. Yes, I'm just being sarcasm..

You don't be sarcasm...