OK.....If You kick the extra point, you have basically 100% chance of being up by 8. The other team has to score a TD, let's assign a 50 percent probability to that. Then they have to convert the two-point conversion, let's assign a 50 percent probablity to that. Then they have to win in OT, let's assign another 50 percent probability to that. So by kicking the extra point, you will win 7/8th of the time.
No, you try for two. 50 percent of the time you basically sealed the victory. The other 50 percent of the time you end up by 7. The other team has a 50 percent chance of tying the game, and then a 50 percent chance of winning in OT. So again you will win 7/8th of the time under that scenario.
Note......The only real downside is that the other team knows they need to twice if you make the two if they are down by 9. They will change their play and make more aggressive plays including on-side kicks and conserve more clock. That probably is only a minor change, but does make a slight case to just go ahead and kick the extra point. It is basically a wash though assuming it is late in the game.