The Bottom Line
When it comes right down to it, the iPad with Retina display and the iPad mini are really more alike than they are different. They run the same software, take the same pictures and have access to the same memory configurations and services for data and cloud. So the buying choice comes down to a few key factors, the foremost being price and main intended usage scenario(s).
For the budget-conscious professional who needs access to email, Web, reader and business applications, the iPad mini will suit that need perfectly well. If more money is available and/or more screen and power are desired for graphics apps, remote desktop or gaming, then the obvious choice would be the iPad 4.