Apologies if this is the wrong group for this question.

The subject says it all. Graphics performance isn't critical. I plan to use programming and not the editor for most of the graphics.

I'm comfortable with both C++, C# and GPU basics so that isn't an issue.

I'd rather use a 3D engine for performance on mobile, allowing quicker scrolling, resizing, etc. Let me know if I'm missing something, but I'm assuming that the mobile GPUs and libraries are not optimized for bit by bit manipulation like GDI+.

Am I missing anything? Are there tools out there that will target PC/iPad/Android using C++?

Unreal Pros

Source code available.

They have an Asset store. It's sparse, but I suspect it will grow quickly.

Unreal Cons

They are just switching models into more of a mass market instead of a boutique business.

I haven't looked into the GUI layer that Unreal has to see how it handles resizable screens, etc. yet.

Unity Pros

Cheaper total cost.

Bigger asset store.

More developers using it.

Unity Cons

I have been looking into using a Unity Graphics layer with a C++ layer for the game logic. Transitioning between the layers is through a C interface will be awkward. Debugging could be more challenging because of this. Performance could be slower because of this.

More expensive up front cost.

Mono is several versions back GC could be slow.

"I have been looking into using a Unity Graphics layer with a C++ layer for the game logic. Transitioning between the layers is through a C interface will be awkward. Debugging could be more challenging because of this. Performance could be slower because of this."

Why would you want to do this? You are crippling the engine's functionality like that. If you start doing it, you will find it's not as straightforward either. And calls to a native code plugin can be slow on iOS.

Unity 2D system is really easy, I don't think Unreal has something embedded.

Unreal can do mobile game, it has a sample on the marketplace, a clone of flappy bird.