The engine does distinguish between various skinning. This is my point about it. Skinning can be less or more expensive, depending on how you do it, how you weight things.
With regards to zero weight, you wouldn’t want that because you want the whole model to move with the bones. All verts should be skinned to some bone at least. And if you had extra bones as markers or something, that’s something else. But all verts should be skinned.
So, I get that it’s a general question, but the answer is based on considering it generally. Like with most things in games, you can use one method or another depending on various other things. So, lets say on much larger scale, you ask the same question, I’d possibly still go with skeletal mesh. But again, it does depend on the project and everything else in it. But I would wager simple rigid skinning above many draw calls, at the base level of the comparison.
So, even in your case, maybe it’s simple and not a real performance question. When and if performance becomes the question, it’s still a close call in my opinion. It’s one of those things where you’d simply have to go and do a large scale test case if you really want to be sure. That’s how you learn what not to do outside of doing it wrong to start with. In my experience though you will use skeletal meshes for animated objects like that more often than not. It just makes more sense.
But just back to skinning. The fact is, it’s cheaper if you have 1 vertex skinned to 1 bone in stead of 1 vertex skinned to 2 or 3 or 4 bones. This is fact, so there should be no confusion there. I will admit that I’m not sure the line there in UE4. In the past, like UDK or other same generation engines, the limit was 4 bones. On PC I will say that it’s less of a concern to use 4 bones. You can get away with a lot more on there. But on mobile it’s more of a concern. Granted, more newer hardware may deal with it better, there’s still many devices out there that will choke real quick.
On the flip side, you have to deal with draw calls, if you use separate meshes. So you have your plane with 2 engines, which is 3 draw calls at best, in stead of 1. Now you add another plane, meaning 6 draw calls in stead of 2. You see where that’s going. Again on PC, you can go quite far, but on mobile, not so much. But still I think draw calls is a bigger concern as a general rule.
Hope that makes sense. What you can do in UE4, open your skeletal mesh, switch on Detailed Info and it will show you how many verts are rigid and how many are soft. So on a character for example, you’ll have a mixture, but on something like a plane, it should be all rigid, which means it will be cheaper for the whole model.