For me this is getting to the point of the Lebron vs Jordan debate.
Yes a flame that burns at 110% for 13 years is absolutely incredible but it's only so long before a flame that burns at 90% for 20 years overtakes the flame that shined oh so bright. At a certain point even the radiance of the brightest flame is outclassed by a flame that just won't quit.
Don't get me wrong, I'm the biggest Steyn fan and he was my favorite bowler growing up, but what Anderson has been able to do for the amount of time he's been able to do it is incredible. This question of who the better test bowler is, is a lot closer now as compared to 5/6 years ago. I would still lean towards Steyn but it's much much closer than what I thought 6 years ago.