What differentiates Brady from Marino?
I'm asking this like you have this huge database and I'm assigned to run a SQL query to support it. And yes, I could crunch the numbers and prove Brady is better than Marino.
I would probably take all QBs and take just their playoff games, then use their winning percentage. Yes, Brady has been in a lot of playoffs, but he's won when he gets there. I don't need to run a query to prove he's better than Romo.
I think you would want something more sophisticated than that. Also factor in 3rd down conversions, percentage of scoring drives, fumbles, interceptions, big plays, etc. This would be an attempt to express in statistics how a Brady wins in a pressure situation, while a Romo loses....both QBs have pretty stats, but there is a way to quantify that gut feeling Brady is clearly better.