There is a good possibility OU and ISU will play each other twice this year and beating OU twice would be tough so if I had to pick a game to lose it would be @ OU then get the win in the Big 12 Championship game because I think ISU would have a tougher time making the CFP losing that last game than OU would if they did. It would be awesome if both teams could get in but it is unlikely to happen which is why it's a razor thin margin ISU has to play with this season and anything short of winning the Big 12 Championship game will still probably get them in a very nice bowl game but would pretty much eliminate them from the CFP with the exception of running the table of an undefeated season going into that game.
Seems like when it comes to picking teams for the CFP or NCAA tournament the last game you play holds the most weight so it's pretty much Big 12 Championship or die for ISU CFP hopes this year IMO.