When referring to the comparison of two things or people then it's always "on par". I think it's safe to say that "on par" is short for "on par with eachother." If you're talking about something being or reaching a particular level, getting to certain point, then you want to say that something is "at par". The phrase "at par" is most commonly heard in golf, explaining that a golfer's score is "at" a certain number, the number that is par. In this case because par is referring to a specific number, you can't really say a number is "on" another number so that's why the preposition used in this is "at".

On par is correct. You say that people or things are on par with each other.

On par

I'd say ... both are on a par