I'd take Warner, and here's why: When it comes to finding a backup, one of those guys will be there. And if Warner is right on, you're in the playoffs. No other QB has as much of a chance to throw 40 this year.
I'd take Warner for both upside and trade value. However, I think Pennington and Hasselbeck are all QB I like that could have #1 QB numbers before the end of the year, and that would likely be sitting around when it's time to pick your #2.
I like Tom Brady and Matt Hasselbeck the most out of that group. I also like Kerry Collins and Tommy Maddox a lot, too, if they are still available. You should get solid numbers at a reasonable price from any of those four.
I have concerns with Brett Favre and Kurt Warner. They won't be making my wish list.