Arm strength= Brady's good here, but he's not the best. Hard to say who is, exactly. There are a lot of guys with cannons, but between Brady and Manning I don't see a clear edge either way.
Accuracy= Pennington might actually be #1 here. Between Brady and Manning, I don't even really care to speculate who's better. They're both really, really good, and if one's better than the other then it's beyond my meager analytical skills to figure it out.
Manning, career: 63% passer outdoors (79 games), 65% passer indoors (97 games).
Brady, career: 63% passer outdoors (103 games), 67% passer indoors (10 games).
Too close to call, IMO. If I was going to give an edge, I'd give it to Brady, since Manning's had better receivers.
Decisionmaking= to clarify, this isn't about reading coverages and all that. It's about, once you've read the coverage, making the right decision with the ball. Analytically, it's hard to separate that from reading coverages, based on what we see as fans, so maybe it's better to wrap them up into post-snap reads. In either case, Brady's lower interception rate and significantly fewer postseason INTs, in particular, suggest that he's the best in the league at this.
Pocket presence= Cassel, despite being more mobile than Brady, got sacked 47 times last year. Brady's pocket awareness is without peer.
Ability to identify and counter blitzes= on one hand, Brady's gone on record as saying he likes being blitzed because it makes his job easier. On the other hand, a couple years ago teams pretty much just stopped blitzing the Colts since they knew how futile it was. Too close for me to call, at least.
Release= Don't know who's best, but between Brady and Manning, Manning's probably better.
Mobility= not coincidentally, none of the best QBs in the league are in the running for this, so you could make a case that it's a minor point, at best.
Taking a hit without going down= probably Roethlisberger. Between Brady and Manning, I'd give it to Manning since Brady's more likely to fumble in these situations.
Anyways, long story short, I'd say that accuracy, pre-snap reads, and post-snap reads are probably the most important of the attributes that we put forward, with pocket presence coming in next, followed by arm strength and quick release.