Sorry, this is my 2 cents without knowing what I'm talking about.... but
I think those are actually 3d. I'm guessing this because you can move around in the game and change your relation to the object, without them following you to only show their 1 side.... well unless they do, and then that's your answer
As for performance; maybe you should just try 3d and see how it goes?
I mean faster is faster, and I would presume that the code running a 2d animation is less expensive than rendering a 3d object (by how much will depend on how many triangles the 3d object needs to draw), but does it really matter? Are you going to blow up mobiles if you go the 3d route, or are you saving on performance when you have tons to spare? If you have hundreds on screen, maybe so, if they are scattered... eh, I dunno.
Another factor to consider: You could actually make and animate a basic 3d coin or other simple object very easily and quickly. I mean you can even just grab a unity sphere, squash it, slap a yellow material with emissions on it, and Bob's your uncle. To animate that to spin or what ever would take you 5 minutes.
On the other hand, animating a 2d object is much much harder. You need to be able to draw and will have to draw the same image (at different angles/stages) a dozen or more times. That requires more skill and much more time.
Well, unless you buy one or something.
So that's another thing for you to consider what is right for
your project, not just a project for mobiles. Do you want to spend money? Is one or another option something that you can actually make yourself?
The last thing that comes to mind; is 2d going to suit the rest of the game? When you move in a 3d space, will the coins look like they are rotating their flat face to watch the camera? Does that matter if they do?
So there you go. You asked 1 question and I "helped" by asking you a dozen more.