Mac mini 2012 graphic upgrade for UE4 Unity3D Blender
I actually wouldn't worry about it too much for Unity3D and Blender - both are very efficient.  UE4 seems to be a bit less efficient.

In the past I've really liked NVidia, but on my late 2011 MacBook Pro (with 8G of normal ram) I'm using a AMD Radeon HD 6750M 1024 MB (only 1G graphics!) and am very happy.  Well, happy with Unity3D and Blender that is.  It runs a bit hot sometimes if I have lots of particle effects and such - but I use a fan control software that helps a bunch.  So, I'm thinking either card you've chosen should work for those two apps.

For UE, I'd go to the forums over there.  Last time I used Unreal, I had to run the Windows version and was using Parallels. I was woefully under-powered for that but, I was still able to grade a bunch of students senior capstones (big projects).  So, even my set-up worked.  Just slooooooow. :-)