The average historical conversion rate across the league is 50%, but you're right that if the defense is dominating, it's a bad idea. I think the break-even point is around 38%-39% conversion rate.
The advantage of going for it on the first is information. If you go on the second, you win or lose depending on the outcome because you're out of time and options. If you go on the first, the information you gain gives you the ability to adapt your next choices: if you made it, you kick PAT the second time. If you fail, you can make up for it with another 2-pt conversion.