The argument can be made that the numbers don't matter, and that Luck is better because he has his team at 8-4 and likely to make the playoffs. Not so fast... Griffin's offense scores more points than Luck's, and turns the ball over less - it's not Griffin's fault that the Redskins can't pull off more wins.
Though the Redskins and Colts are 23rd and 24th in scoring defense, the Colts' number is skewed by a couple of blowout losses. The Colts' defense has given up an average of 18.6 points per game in their eight wins - that number would ranked 5th in the league if they maintained it through their four losses. They've held opponents to 20 points or less in six of their eight wins. Luck hasn't had to put too many points on the board to win football games.
The Redskins have given up 22.2 points per game in their wins (and that's dragged down by a 31-6 win over the Eagles), which would rank 17th in the league if they maintained it in through their six losses, so Griffin has had to do more than Luck to earn his victories.
One final note: Griffin has almost no weapons to throw to. Pierre Garcon has been pretty good when on the field and healthy, but he's only played in six of the team's games and has been gimpy in a few of them. Luck's No. 1 receiver is a surefire Hall of Famer, and the receiving options around him (Avery, Hilton, etc.) are better than a washed up Santana Moss and some guy named Leonard Hankerson.
Don't get me wrong, I think Andrew Luck is exceptional and is having a historic season as a rookie quarterback, but Robert Griffin III would be having a historic season even if he wasn't a rookie. I can't think of a quarterback who ever ran like Griffin does while maintaining a quarterback rating of 100+. What RG3 is doing is ridiculous, and he may have a worse record than Luck, but I think he's had a more impressive season. Plus, the Redskins are one game back of the NFC East leaders with three of their final four games are against teams that have .500 records or worse, so absolutely do not count RG3 out of the postseason.