The thing about trading for the next year's #1 is that you never know where THAT is going to land. Look at the Pats. They're going to be in a worse situation this draft, pick-wise, than they were last year. If the Ravens make the playoffs, as well, they're going to have two sub-20 1st rounders.
If the Raiders wanted to trade and drafted Eli and then sat him and stayed with Mirer or Gannon, they could easily go 8-8, and the Cards would be in the same situation as the Saints were last season, with two mid-round useless picks.
I think that you have to go with the surest thing that you have. I don't think that this team lacks talent as much as catalysts that the team can rally around on each side of the ball, like Ray Lewis in Baltimore or Michael Vick in the ATL. Those kinds of players raise the game of no only their side of the ball, but THE ENTIRE TEAM.
I just don't see that kind of defensive player in this draft. Julius Peppers was the last defensive one that I remember. And while we can use a corner, I don't really think that they can play that role (look at Champ Bailey or Aeneas Williams when he was here; did he elevate the play of our entire defense?). Corners in the Top 5 or 10 are as risky a prospect as wideouts; ask San Diego how they like Quentin Jammer.
Eli Manning is the most solid option that the Cards have at #1. Take it.