The biggest difference between Rex and Griese is Rex NEEDS to have really good widerecievers. Im talking about a sure go to guy that will stretch the field and catch the deep passes. Brian may get more yardage than Rex as of now because he sees the field totally different than Rex does. He takes things more slowly and dumps it off to the Running back who usually gets like 8-12 yards when Rex would rather just pass it for the 12-15 yards right away.
If our Recievers establish themselves as being a great core then Rex will succeed. If not, then he will be nothing more than a poopstain in the underpants of society.