I think a lot of this will boil down to timing/sequencing.
If Manning is considered by the Cards to be a real possibility, then the smart move would be to table Kolb & Skelton until the Manning situation is resolved.
If we don't sign Manning, it then comes down to Kolb's cap hit vs. both Kolb's and Skelton's perceived talent (both comparatively and in the absolute). If there's a deadline to decide on paying Kolb and taking a cap hit, then that's the next bridge to be crossed.
If we do sign Manning, same deadline applies, but most likely, we don't sign Kolb.
Meanwhile, Skelton (despite the fact that he could emerge as starter) has to sit and wait.
But if, for some reason, the Cards decide that the cap hit by signing or not signing Kolb is moot, I could see them taking the luxury of taking their time to see which of Kolb or Skelton looks best as we move thru mini-camps, OTA's and training camp (i.e why rush a decision when you don't have to.
Bottom line - time sequence figures to matter here.