In a vacuum, the simple math agrees with you. All else equal, go for two. But, that works long-term over several games. But, are coaches enough to put up with rough spots over time as the math "evens out?"
We all know tackles aren't made in a vacuum. Some teams defend 2-pts better than others. Some teams won't adopt a similar mindset, so if you score a touch and are down 7-6, you have to assess if the above as well as how many more swings are you going to get at TD if you don't convert. Also, how dinged up is your offense? How good is your kicker?