Go retina and you'll never regret it or go back.
I was in my local Apple Store today and spent a long time with the iPad3's retina display. I've only briefly seen it on a friend's iPad3. For me the retina screen's quality confirmed my decision to upgrade from an iPad2 to a 3. The iPad4's other features are frosting on the cake!
My other realization about the retina display is that it has to be on my next Macbook Pro. I also wish the Air had the retina.