The iPad 4 is definitely faster, the screen looks better, etc., but the point is you want the form-factor of the mini or you want the form-factor of the regular iPad.
If you compare spec for spec, then the mini will look and feel horrible.
I spent about $800 (with tax and AppleCare+) on the iPad mini, so really it was down the fact that the smaller iPad is something that I want, not the regular sized iPad (which I also own, but have not touched once since I bough the mini).
I don't miss the Retina display and the iPad is fast enough for everything I need to do on it.
Hell, I didn't buy my rMBP for the Retina display. I wanted a thinner 15" MacBook Pro.