You need to compare them to their own position to compare them.
McGahee = 34 pts greater than the 24th RB
Manning = 21 pts greater than the 12th QB
Appears McGahee is the better chioce at this time.
The QBs that could have been had in the 4th round or later that are producing near Manning.....Brady, Eli Manning, Collins, Brunell, Palmer
Your team would have been better if you drafted a WR or RB in the 1st round and got one of those QBs.
It depends on your league and scoring system. In the league where I picked Manning in the first round, it rewards heavily for TD's and has very tight rosters. As a first-year dynasty league, converting from redraft, those roster spots are extremely valuable. I'm very happy with the Manning pick now, having only used 1 roster spot for QB except during Manning's bye. Since we can't keep any picks from the first 3 rounds, not having to use any roster space on a QB after round 3 is especially valuable.Despite his slow start, Manning has outscored the 12th QB by over 30 points, and he's only recently been coming on strong. McGahee has outscored the 24th RB by 27, and he's been fading recently. My 8th round RB, Stephen Davis, has outscored McGahee by 15 points. You can't say that about any QB in our league. Only Palmer and McNabb have outscored Manning. No way I'd have predicted Palmer's production, and McNabb will be out several weeks now. My 5th round RB, Larry Johnson, will likely be the top RB from here on out.
So, in this case, I used 1 roster spot for a consistent stud QB who has outscored most of the 1st-round RB's in this scoring system on a relative value basis, and have used the extra roster space to get extra RB's, which allowed me to get Davis and LJ late. Meanwhile, the Culpepper, McNabb, Bulger, Green, and Vick drafters (the next 5 QB's taken) are hurting, with McNabb out and the rest 40+ points behind Manning.
Sure, you can pick out Palmer and say Manning's a bust because a later round pick is outperforming him, but you could do the same with any 1st round RB other than LT, Alexander, and Edge, all of whom are more likely to get injured than Manning. In this league, the 2nd through 6th QB taken are much more busts than Manning has been, even if Manning were taken 1st overall. Drafting Manning allowed me to lock down that position without having to guess which of the later round QB's would outperform expectations and then draft at least 2 of them. That allowed me to use my edge in fantasy football knowledge on later RB's like LJ and Davis. Now I have likely the best keeper in LJ.
Why some people continually pontificate on how Manning was a bad early pick without respect for different leagues' different scoring systems and roster spaces is beyond me. Even with Manning's early disappointments, he's still been a solid 1st round value in my league.