Why don't you want to compare the records of both teams since their inception?
You can't because the Cowboys didn't exist until 1960.
However, if we use All Time stats, the Steelers still dominate the Cowboys. All Time, the Steelers have 625 wins, the Cowboys have 521
All Time, the Steelers are 34-23 in the playoffs, the Cowboys are 34-26. Therefore, the Steelers have a better playoff percentage than the Cowboys.
All Time, the Steelers have 6 championships, the Cowboys have 5
All Time, the Steelers have beaten the Cowboys in 2 of 3 Super Bowls.