How many points do you think that you need from your QB to win your matchup?? Kolb is more of a high risk/high reward option while Freeman will give you decent points for sure.
If you think that with decent QB points you'll win, start Freeman. If you think that you'll need more than decent points to win your matchup start Kolb.
Anyway, I would start Kolb because theres not a lot of "high risk" situation playing against the Panthers' D and Freeman will not give you more than decent points even playing against the Lions.