Lag or Larger form factor....
If you don't mind lag in the most demanding games for iOS go with the Mini. If you want the best buttery smooth high-resolution experience, go with the iPad 4.
We own both in my home. The mini is perfect for angry birds, temple run etc.....
For Real Racing, Nova 3, MC3, Kingdom Rush(screen real estate), Asphalt 7 etc....the A6X owns.
I know those who have an iPad 3 or 4 will be glad they do, the day Real Racing 3 releases.