Why?
I mean, it's all judgement calls, but in my view, I'd compare Brady and Manning as follows:
Arm strength, Manning wins hands down.
Accuracy, it's a toss-up.
Clutch/Intangiles, absolutely goes to Brady.
Poise in the Pocket, you have to give this to Brady.
Reading defenses, I would say is even as well.
So I wouldn't say Brady is head-and-shoulders above Manning, but I would prefer to have him over Peyton if I had the luxury of choice.