Holmes in a landslide. The guy has been incredibly productive and has great combine stats in terms of speed. Jackson, on the other hand, has not had a great college career. He's jumped from being a 2nd/3rd round prospect to being a high first on the strength of ONE 40 YARD TIME! To me that's just completely nuts, and you take a big risk of a bust if you pick him up high. Maybe it's just because I think the combine is INCREDIBLY overhyped, as if a single workout can really give you a more accurate depiction of a player's skills, athleticism and value than their entire college career.
Put it this way: the combine measures athleticism (in a flawed kind of way). We aren't just looking for athletes, we want football players. It may be that Jackson is a more impressive athlete than Holmes, but I think we can all agree that athelticism isn't enough in the NFL (otherwise every super-hyped combine workout warrior would be a superstar). But Holmes is simply a better football player, so he gets my vote.