Very interesting debate, but in all honestly an impossible one to win. Playing quarterback and playing defensive line are almost like two different games. "D-Linemen get hit every play, they are tougher than quarterbacks." "Sure, a defensive lineman gets hit every play, but does he ever get hit with his arms in the air and his ribs exposed?"
As a former quarterback I think you have to be tougher to play quarterback. The severity of shots you can and will take, unexpectedly, is scary. You are literally a target for these defensive linemen. Their goal on roughly half of the plays is to intimidate and pound the opposing quarterback.
Now, that is not to say it takes some serious balls to play defensive line. I think, apart from running backs, defensive linemen are probably the most likely to experience head trauma, at least later in life. They experience repetitive blows to the head. Now they may not be severe all the time, but play in and play out they are getting whacked. But are they ever asked to have a blitzing linebacker run through them full speed when they aren't ready? I don't think so. And then expected to bounce back like nothing happened?
Quarterbacks are tougher than d-linemen, agreed?