In this hypothetical we can go back and forth in time in Doc Brown's DeLorean, so that both Grogan and Plunkett are current-day rookies. That means they both have the same advantages of current day training, nutrition, coaching, medicine, technology, etc.
Plunkett is the more accurate passer with greater arm strength. Grogan provides an added dimension as a threat to run the ball.
Who do you want as your quarterback: a player that is more of a pure pocket passer, along the lines of a Matt Ryan? Or is a player more similar to Russell Wilson your preference?
Grogan always has been and always will be one of my favorite players. Despite Grogan's slight edge for his toughness and leadership, I would go with Plunkett though. He is the superior pocket passer that could thrive in today's NFL as long as he had a decent offensive line and coach/front office.